AP Chemistry Question of the Day --41

Welcome back to my blog. Here comes the answer to question 40 I posted yesterday. The correct answer is C. 

Assume we have 100g of the sample, then we will have 48.5g oxygen and 51.5g vanadium. 

V and O has a 1:3 ratio. As a result, the empirical formula of the oxide is VO3.

Following is Question 41 of the series. 

41. When 0.100 kg of solid metal at 95.0 °C is placed into 0.300 kg of water at 25.0 °C, the final temperature of the system is 27.4 °C. What is the identity of the metal? Assume the system is thermally insulated from its surroundings. 

A Fe(s) 
B Sn(s) 
C Mn(s) 
D Ti(s) 
E Pb(s) 

I will post the answer and explanation tomorrow. 

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AP Chemistry Question of the Day --40

Welcome back to my blog. Here comes the answer to question 38 I posted yesterday. The correct answer is C. 

First let's refresh our minds with the rules of writing Lewis structures.

1. —Determine the central and terminal atoms

2. Determine the total valence electrons in molecule or ion

—3. Form single bond between atoms.

—4. Any electrons remaining are placed on terminal atoms first to meet octet rule.

—5. Form double or triple bond if the central atom has less than eight electrons. If B is the central atom, it can have less than 8 electrons. —The central atom may have more than 8 electrons only if it is in period 3-7. The max. number of electrons is 12.

From the question, it is stated that S is the central atom. The total numbers of valence electrons are 6+6+7+7=26. 6 are used to make the S-O and 2S-Cl bonds. 18 will be used to make all the terminal atoms octet. The two electrons that are left over are then placed on the central S atom. Now we count the number of electron groups around S which is 4. So The central S takes tetrahedron geometry and the bond angle would be close to 109. C is the correct answer. 

Following is the Lewis structure for SOCl2:

Following is Question 40 of the series. 

40. 

I will post the answer and explanation tomorrow. 

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AP Chemistry Question of the Day --39

Welcome back to my blog. Here comes the answer to question 38 I posted yesterday. The correct answer is B. 

Following is Question 39 of the series. 

39. 

I will post the answer and explanation tomorrow. 

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AP Chemistry Question of the Day --38

Welcome back to my blog. Here comes the answer to question 37 I posted yesterday. The correct answer is D. 

When writing net ionic reactions, the following needs to be kept in mind:
1. If the substance is a strong electrolyte, it dissociates in water completely. We write them as dissociated ions. All soluble ionic compounds are strong electrolyte. Although HCl, HBr and HI are covalent compounds, they are strong acid and dissociate completely in water as well. 
2. If the substance is a weak electrolyte, it barely dissociate in water and we write it in its molecular form. All covalent compounds except HCl, HBr and HI are weak electrolyte, including water itself.
3. If the substance is a salt but is not soluble in water, we write it in molecular form too because it is mainly staying in the solid state as a solid.
4. If an ion is on both side of the equation, it is called a spectator ion and it should be cancelled out and not appear in the final net ionic equation. 

Now we look at our question. HClO is a weak acid as stated in the question. Ca(OH)2 is a strong electrolyte. the product Ca(ClO)2 is also a strong electrolyte. Water is a weak electrolyte. As a result the reaction can be written as:

Calcium ion appears on both sides of the equation, we cancel it out and the final net ionic reaction is:

This is answer D.

Following is Question 38 of the series. 

38. 

I will post the answer and explanation tomorrow. 

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AP Chemistry Question of the Day --37

Welcome back to my blog. Here comes the answer to question 36 I posted yesterday. The correct answer is D. 

When writing ground state electronic configurations, we have to follow several rules:
1. Aufbau Principle: Electrons fill the lowest energy orbitals first.  The order of energy level is: 1s 2s 2p 3s 3p 4s 3d 4p 5s 3d 5p 6s 4f 5d 6p 7s 5f 6d 7p .....
2. Pauli Exclusion Principle:  In a given atom, no two electrons can have the same set of 4 quantum number. This rule determined that each orbital can hold only two electrons.
3. Hund Rule: —The lowest energy state of an atom is the one having the max numbers of unpaired electrons. When electrons are filling degenerate orbitals, they tend to occupy all the orbitals first and then pair up if they have to.

No    Now we see, The electron configuration of the atoms are:

         

According to the Hund Rule, if a sublevel is not fully filled, electrons tend to occupy all the orbitals with the same energy level first and then pair up if necessary. Li has 1, B has 1, Sc has 1, V has 3 and Ni has 2 unpaired electrons respectively.

Following is Question 37 of the series. 

37. What is the net ionic equation for the reaction that occurs when aqueous solutions of HOCl, a weak acid, and calcium hydroxide, Ca(OH)2, are combined? 

I will post the answer and explanation tomorrow. 

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AP Chemistry Question of the Day --36

Welcome back to my blog. Here comes the answer to question 35 I posted yesterday. The correct answer is C. 


The above reaction can be the sum of the following two reactions:

So, C is the correct answer.

Following is Question 36 of the series. 

36. Which of the following has three unpaired electrons in its lowest energy state? 

A Li 

B B 

C Sc 

D V 

E Ni 

I will post the answer and explanation tomorrow. 

Thank you very much for reading my post, I am available to discuss any topics in AP chemistry. Please give me a call at 604-808-6827 or email me at vancouverchemistrytutor@gmail.com. My website is located at www.vancouverchemistrytutor.com.

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AP Chemistry Question of the Day --35

Welcome back to my blog. Here comes the answer to question 34 I posted on Friday. The correct answer is A. 

Adding something that reacts with Co2+(aq) would decrease the concentration of Co2+ and hence shift the equilibrium toward the right to offset the disturbance.

Since the reaction is exothermic, increasing the temperature of the reaction will shift the equilibrium to 
the left.

Adding a fluoride salt will increase the concentration of fluoride, which is a product of the reaction and will shift the equilibrium to the left.

Adding a catalyst will not shift the equilibrium. Catalyst can only make it faster to reach equilibrium.

Decreasing the volume of the reaction will shift the equilibrium towards the direction where less gas molecules are. In this case, it is shifting to the left.

So, A is the correct answer.

Following is Question 35 of the series. 

35. Given the following reactions and equilibrium constants:

what is K for the reaction shown below? 

 I will post the answer and explanation tomorrow. 

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AP Chemistry Question of the Day --34

Welcome back to my blog. Here comes the answer to question 33 I posted yesterday. The correct answer is B. 

Following is Question 34 of the series. 

34. The following exothermic reaction reaches equilibrium in a closed reaction vessel, 

Which of the following actions causes net reaction from left to right? 
A adding something that reacts with Co2+(aq) 
B increasing the temperature 
C adding some soluble fluoride salt, such as NaF(s) 
D adding a catalyst 
E decreasing the volume of the reaction vessel 

I will post the answer and explanation tomorrow. 

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AP Chemistry Question of the Day -33

Welcome back to my blog. Here comes the answer to question 32 I posted yesterday. The correct answer is B. 

The sum of oxidation numbers of the individual element should be equal to the total charge of the particle. Since all the available choices are neutral, the sum of oxidation number should be zero. K always has +1 oxidation state in its compounds. Oxygen normally has an oxidation state of -2 except in peroxide compounds where its oxidation state is -1. Halogens always has oxidation state of -1.

Thus, Mn's oxidation states are +4, +2, +6, +7 and +7 in the five choices.  Answer B has the lowest.


Following is Question 33 of the series. 

33. How many moles of K+ ions are there in 0.12 L of 0.015M  K2SO4(aq)? 

I will post the answer and explanation tomorrow. 

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AP Chemistry Question of the Day --32

Welcome back to my blog. Here comes the answer to question 31 I posted yesterday. The correct answer is E.

When pressure is high and temperature is low, we can consider gases as ideal gas and they obey the ideal gas law. The molar volume of any gas would be the same at the same pressure and temperature according to PV=nRT, the molar volume (n=1)would be: V(molar)=RT/P. Under ideal gas conditions, the volume and the intermolecular interactions between gas molecules are negligible. However, when temperature is low and the pressure is high, especially when the gas is close to condense, both the volume and the intermolecular forces have to be taken into account. The stronger the intermolecular force, the less the volume. Both C2H6  and H2 are non polar molecules. Since C2H6 has higher molecular weight and more complicated electronic structure, it has stronger London Dispersion forces than H2. So E is the right answer.

Answer A is incorrect because C2H6 is larger than H2. The average kinetic energy is only dependent on the temperature. At the same temperature,  both gases have the same average kinetic energy. Thus, B and C are wrong. 



Following is Question 32 of the series. 

32. In which compound does manganese have the lowest oxidation state? 

A MnO2 
B MnI2 
C K2MnO4 
D Mn2O7 
E KMnO4 

I will post the answer and explanation tomorrow. 

Thank you very much for reading my post, I am available to discuss any topics in AP chemistry. Please give me a call at 604-808-6827 or email me at vancouverchemistrytutor@gmail.com. My website is located at www.vancouverchemistrytutor.com.

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AP Chemistry Question of the Day --31

Welcome back to my blog. Here comes the answer to question 30 I posted yesterday. The correct answer is C. 

Water is a very polar molecule while hydrocarbons of high molecular weight are unpolar compounds. According to "Like dissolves like" Rule, The solubility of high molecular hydrocarbons will be very low in water.
Due to hydrogen bonding, water has a higher density as a liquid than as a solid, because the lattice structure in ice restricted the number of hydrogen bonding while in liquid water, the maximum hydrogen bonding could be achieved. Thus A and B are correct. Answer D is also correct because when we write the Lewis structure of water, the central oxygen atom has two bonds connected to hydrogen and two lone pairs of electrons. The central oxygen atom is sp3 hybridized and takes tetrahedron geometry. As the two lone pair electrons are not visible, the molecule appears as a bent shape.

Following is Question 31 of the series. 

31. Which of the following statements accounts for the observation that, at 300 K and 10 atm, the molar volume of C2H6(g) is less than that of H2(g)? 

A C2H6 molecules are smaller than H2 molecules. 
B The average kinetic energy of H2 molecules is greater than that of C2H6 molecules. 
C The average kinetic energy of H2 molecules is less than that of C2H6 molecules. 
D The intermolecular attractions in C2H6(g) are weaker than they are in H2(g). 
E The intermolecular attractions in C2H6(g) are stronger than they are in H2(g). 

I will post the answer and explanation tomorrow. 

Thank you very much for reading my post, I am available to discuss any topics in AP chemistry. Please give me a call at 604-808-6827 or email me at vancouverchemistrytutor@gmail.com. My website is located at www.vancouverchemistrytutor.com.

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AP Chemistry Question of the Day --30

Welcome back to my blog. Here comes the answer to question 29 I posted yesterday. The correct answer is D. 

Activation energy is the energy required to form the activated complex of the reaction so that effective collision can occur. The activation energy of a particular reaction is only dependent on the nature of the reaction. Thus, A is incorrect. The spark supplies some of the energy to form the activated complex of the reaction. Since the reaction is exothermic, once the reaction is started, the heat released can keep the reaction going.


Following is Question 30 of the series. 

30. Which statement about the properties of water is incorrect? 

A Water expands as it freezes. 
B The density of water is greater than that of ice. 
C Water is a good solvent for hydrocarbons of high molecular weight. 
D Water molecules are V-shaped (bent). 

I will post the answer and explanation tomorrow. 

Thank you very much for reading my post, I am available to discuss any topics in AP chemistry. Please give me a call at 604-808-6827 or email me at vancouverchemistrytutor@gmail.com. My website is located at www.vancouverchemistrytutor.com.

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AP Chemistry Question of the Day --29

Welcome back to my blog. Here comes the answer to question 28 I posted yesterday. The correct answer is B. 

When an acid molecule loses its proton, it becomes a base.The acid and base are referred to as a conjugate acid-base pair. The conjugate acid has one more hydrogen than the conjugate base. Thus, B is the right answer. 

Answer C is not correct because it is one hydrogen less than ammonia and it is the conjugate base of ammonia. Answer D is incorrect because it is one electron less than ammonia. 



Following is Question 29 of the series. 

29. Which of the following best describes the role of a spark in a butane lighter? 

A. The spark decreases the activation energy of the combustion reaction.

B. The spark increases the concentration of butane in the reaction chamber.

C. The spark provides the heat of vaporization for the liquid fuel.

D. The spark supplies some of the energy to form the activated complex for the combustion reaction. 

I will post the answer and explanation tomorrow. 

Thank you very much for reading my post, I am available to discuss any topics in AP chemistry. Please give me a call at 604-808-6827 or email me at vancouverchemistrytutor@gmail.com. My website is located at www.vancouverchemistrytutor.com.

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AP Chemistry Question of the Day --28

Welcome back to my blog. Here comes the answer to question 27 I posted yesterday. The correct answer is B. 

This is a kinetic problem asking the order of the reaction with respect to a reactant. We need to know:

• If the reaction is zero order, then the plot of concentration or pressure in the case of gas vs time is linear.
• If the reaction first order, then the plot of ln(concentration or pressure in the case of gas)  vs time is linear.
• If the reaction is second order, then the plot of the reciprocal of concentration or pressure in the case of gas is linear.

From the data given in the table we know, the ln P(N2O5)  vs time is linear. As a result, the reaction is first order and B is the right answer.

Following is Question 28 of the series. 

28. Which of the following is the conjugate acid of NH3?

I will post the answer and explanation tomorrow. 

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AP Chemistry Question of the Day --27

Welcome back to my blog. Here comes the answer to question 26 I posted yesterday. The correct answer is A. 

NaCl is an ionic compound. The melting points  and boiling points of ionic compounds are determined by the strength of Coulombic attractions between the cation and the anion. The higher charge the ions carry and the shorter distance between the ions, the stronger the Coulombic attraction. 

Since Cl ion has a larger radius than F ion,  the distance between the ions are longer in NaCl and thus the attraction is weaker in NaCl. NaCl will have a lower boiling point compared to NaF. Answer A is right. 


Following is Question 27 of the series. 

27.

A sample of N2O5 was placed in an evacuated container, and the reaction represented above occurred. The value of P(N2O5) , the partial pressure of N2O5(g), was measured during the reaction and recorded in the table below.

Which of the following correctly describes the reaction?
(A) The decomposition of N2O5 is a zero-order reaction.
(B) The decomposition of N2O5 is a first-order reaction.
(C) The decomposition of N2O5 is a second-order reaction.
(D) The overall reaction order is 3.

I will post the answer and explanation tomorrow. 

Thank you very much for reading my post, I am available to discuss any topics in AP chemistry. Please give me a call at 604-808-6827 or email me at vancouverchemistrytutor@gmail.com. My website is located at www.vancouverchemistrytutor.com.

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AP Chemistry Question of the Day --26

Welcome back to my blog. Here comes the answer to question 25 I posted yesterday. The correct answer is C. 

First of all, we have to know that this question is asking about trend in physical properties, boiling point particularly in this case. Then we need to determine what kind of forces is in play. Since both compounds are covalent molecules. It is the strength of intermolecular force that decides the trend in boiling point. Both compounds are non polar compounds. The only intermolecular force in non polar molecules is London Dispersion Force. The strength of the London Dispersion Force is dependent on the molecular weight and the complexity of the compound. In the same series, the higher the molecular weight, the stronger the intermolecular force. When the molecular weight are comparable,  the more complex the molecular structure, the stronger the London Dispersion Force. Thus, the two molecules in question have almost the same molecular weight, nonane has a longer carbon chain, which means a more complex structure will have higher boiling point.

Answer A and B are not correct because during the precess of evaporation, no covalent bond is broken.   Boiling point has no direct relationship with the polarity and strength of the covalent bond. Answer D is not correct because when a compound has higher boiling point, the intermolecular force is stronger which held the molecules closer, not further apart.

Following is Question 26 of the series. 

26.

Based on the data in the tables above, which of the following statements provides the best prediction for the boiling point of NaCl?

(A) NaCl will have a lower boiling point than NaF because the coulombic attractions are weaker in NaCl than in NaF.

(B) NaCl will have a boiling point between that of NaF and MgO because the covalent character of the bonds in NaCl is intermediate between that of MgO and NaF.

(C) NaCl will have a higher boiling point than MgO because the ions are spaced farther apart in NaCl.

(D) NaCl will have a higher boiling point than MgO because the energy required to transfer electrons from the anion to the cation is larger in NaCl than in MgO.

I will post the answer and explanation tomorrow. 

Thank you very much for reading my post, I am available to discuss any topics in AP chemistry. Please give me a call at 604-808-6827 or email me at vancouverchemistrytutor@gmail.com. My website is located at www.vancouverchemistrytutor.com.

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AP Chemistry Question of the Day --25

Welcome back to my blog. Here comes the answer to question 24 I posted yesterday. The correct answer is D. 

From the spectra, it shows that 550eV energy is needed to remove one 1s electron from oxygen, while only 400eV is needed to remove one 1s electron from nitrogen. Since there is static attraction between the nuclear and electrons. The stronger the attractive force, the higher energy it needs to remove the electron to overcome the attraction. The strength of the static force is determined by the charge on the nuclear and the distance. In both cases, the electrons are located in the 1s shell, which means that the distances between the nuclear and the electron are about the same in O and N. The difference lies in the nuclear charge. N has 7 protons and O has 8 protons in the nuclear. As a result, O nuclear has a stronger attraction towards the electron and it takes more energy to remove the 1s electron from a oxygen atom than the 1s electron from a nitrogen atom.

Following is Question 25 of the series. 

25. 

Consider the molecules represented above and the data in the table below.

Nonane and 2,3,4-trifluoropentane have almost identical molar masses, but nonane has a significantly higher boiling point. Which of the following statements best helps explain this observation?

(A) The C–F bond is easier to break than the C–H bond.

(B) The C–F bond is more polar than the C–H bond.

(C) The carbon chains are longer in nonane than they are in 2,3,4-trifluoropentane.

(D) The carbon chains are farther apart in a sample of nonane than they are in 2,3,4-trifluoropentane

I will post the answer and explanation tomorrow. 

Thank you very much for reading my post, I am available to discuss any topics in AP chemistry. Please give me a call at 604-808-6827 or email me at vancouverchemistrytutor@gmail.com. My website is located at www.vancouverchemistrytutor.com.

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AP Chemistry Question of the Day --24

Welcome back to my blog. Here comes the answer to question 23 I posted yesterday. The correct answer is C. 


Metallic solids are often pure substances, but may also be mixtures called alloys. Some properties of alloys can be understood in terms of the size of the component atoms:
— Interstitial alloys form between atoms of different radius, where the smaller atoms fill the interstitial spaces between the larger atoms. The interstitial atoms make the lattice more rigid, decreasing malleability and ductility, increasing hardness as seen in this question. This is the case of Au and Cu alloy.
— Substitutional alloys form between atoms of comparable radius, where one atom substitutes for the other in the lattice. The density typically lies between those of the component metals, and the alloy
remains malleable and ductile. This is the case for Ag and Au alloy.

 Following is Question 24 of the series. 

24. The photoelectron spectra above show the energy required to remove a 1s electron from a nitrogen atom and from an oxygen atom. Which of the following statements best accounts for the peak in the upper spectrum being to the right of the peak in the lower spectrum?

(A) Nitrogen atoms have a half-filled p subshell.
(B) There are more electron-electron repulsions in oxygen atoms than in
nitrogen atoms.
(C) Electrons in the p subshell of oxygen atoms provide more shielding than
electrons in the p subshell of nitrogen atoms.
(D) Nitrogen atoms have a smaller nuclear charge than oxygen atoms.

I will post the answer and explanation tomorrow. 

Thank you very much for reading my post, I am available to discuss any topics in AP chemistry. Please give me a call at 604-808-6827 or email me at vancouverchemistrytutor@gmail.com. My website is located at www.vancouverchemistrytutor.com.

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AP Chemistry Question of the Day --23

Welcome back to my blog. Here comes the answer to question 22 I posted yesterday. The correct answer is C. 

The Energies of visible and ultraviolet light photons are comparable with the electron energy level differences in atoms and molecules. When electron is excited from ground state or lower energy level to a higher energy level, energy needs to be absorbed. When electron goes from a higher energy level to a 

lower energy level, energy will be released. Thus, C is the correct answer.

When molecules vibrate, they absorb energy from the infrared region. So, answer A is not correct. Answer B is related to ionization energy. Molecular mass has no direct relationship with energies. Both B and D are incorrect.

 Following is Question 23 of the series. 

23. 

To make Au stronger and harder, it is often alloyed with other metals, such as Cu and Ag. Consider two alloys, one of Au and Cu and one of Au and Ag, each with the same mole fraction of Au. If the Au/Cu alloy is harder than the Au/Ag alloy, then which of the following is the best explanation based on the information in the table above?

(A) Cu has two common oxidation states, but Ag has only one.
(B) Cu has a higher melting point than Au has, but Ag has a lower melting point than Au has.
(C) Cu atoms are smaller than Ag atoms, thus they interfere more with the displacement of atoms in the alloy.
(D) Cu atoms are less polarizable than are Au or Ag atoms, thus Cu has weaker interparticle forces.

I will post the answer and explanation tomorrow. 

Thank you very much for reading my post, I am available to discuss any topics in AP chemistry. Please give me a call at 604-808-6827 or email me at vancouverchemistrytutor@gmail.com. My website is located at www.vancouverchemistrytutor.com.

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