AP Chemistry Question of the Day--68

I was tutoring a student and came across the following question, I found it to be a good question regarding several concepts in chemistry.

A 2cm Mg ribbon was reacted with 100ml 0.10M HCl and at the end of the reaction, all the Mg ribbon has dissolved. The temperature of the reaction mixture increased by 20 degrees. Another 1cm Mg ribbon was reacted with 0.1M HCl in a separate reaction. How much HCl solution is required to make the temperature increase 10 degrees?

A. 50mL
B. 100mL
C. 200mL
D. Could not be decided.

The correct answer is B.
Mg reacts with HCl and produces MgCl2 and Hydrogen gas. If at the end of the reaction, all the Mg has dissolved, it means that Mg was the limiting agent. If half the length of Mg was used in the second experiment, then the amount of heat released should also be halfed.
According to q=mc(deltaT), both the q and delta T were half of the original, the m should stay the same. m here is the mass of the total reaction mixture. If we assume the reaction mixture has the same density as water, then we can deduce that the mass of the reaction mixture depends on the volume of the HCl solution. As a result, the volume of the HCl solution should remain as 100mL.
A common mistake would think the m in q=mc(deltaT) refers to the mass of the reactants. Because the temperature increase occurs to the overall reaction mixture, the mass should always be the mass of the total reaction mixture, not the mass the reactants themselves.

Hope this helps with your AP review. Good luck in your exam in May.

AP Chemistry Question of the Day --67

It has been a while since I last posted an AP Chem question. I recently got an email acknowledging me that this blog is helpful. I am glad to hear it and decided to resume it. Here comes question 67.

The equilibrium constants for the following reaction:

N2(g) +O2(g) ⇄2NO(g)

are Kp=1.1e^3 at 2200K and Kp=3.6e^3 at 2500K respectively. Which of the following statement is true:

A. Higher total pressure shifts the equilibrium to the left.
B. The partial pressure of NO(g) is less at 2200K than at 2500K
C. The total pressure at 2200K is the same as at 2500K
D. The reaction is exothermic
E. Kp is less than Kc by (RT)

The correct answer is B.

Since both the reactant side and the product side have 2 moles of gas. The change in pressure or total volume will not shift the equilibrium, which eliminate A.
C sounds right but is actually wrong because as the total mole of the gases and the volume of the gases stay unchanged, the total pressure will increase as a result of the temperature change. The total pressure should be higher at 2500K than at 2200K.
As the temperature increases from 2200K to 2500K, the equilibrium constant increases, indicating the  forward reaction is endothermic.
Since there are equal number moles of gases on both sides, the Kp should be equal to Kc

Thank you for visiting my blog. Let me know if you have any particular topics or questions by email me at vancouverchemisrtytutor@gmail.com

AP chemistry of the day --66

This year's AP exam is done on May 1st. Hope all my students did well. The results will be out at the beginning of July. As always, I am less busy and will continue with my blog.
Here is the 66th question of my series.

66. The ionization process of water is endothermic with Kw=1.0e-14 at 25 degrees Celsius. What is the pOH of a solution that has a pH of 2 at 50 degrees Celsius?


A. greater than 12
B. Less than 12
C. equal to 12
D. can not be determined

The Kw of water is a special case Keq which represents the Keq of the self ionization process of water. All the properties of Keq apply to Kw. Since it is an equilibrium constant, it only depends on temperature. Since the process is endothermic, Kw will increase as temperature increases. As a result, at 50 degrees Celsius, Kw will be less than 1.0e-14. In neutral water, the concentration of hydronium ion and hydroxide ion are always equal, pH=pOH and the. sum of pH and pOH is always pKw. pKw at 25 degree Celsius is 14. At 50 degrees Celsius, both ion concentration will be less than 1.0e-7. the total of pH and pOH will be less than 14. if pH is 2, then pOH will be less than 12.
B is the correct answer.

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AP Chemistry Question of the Day --65

Long time no see. It has been a while  since I last wrote on my blog. The 2016 AP exam is done and I personally think this year the free response questions are easier than last year. I would expect a higher  percentage of 5 overall.
Here comes Question 65 of the series:

Q 65: Which of the following factors would affect vapour pressure:
I. Identity of the substance;
II. Temperature
III. Shape of the container
IV. Size of the container
A. I and II
B. I and III
C. I, II and III
D. all of them

The correct answer is A. Vapour pressure indicates a substance's tendency to evaporate. substance with higher intermolecular force tends to have lower vapour pressure. Also the vapour pressure is only dependent on the temperature for a given substance and size and shape of the container will not affect vapour pressure since it represents the ability of the substance to evaporate. Pressure itself is an intensive property.

Let me know if you have a particulate question regarding AP chemistry or any question regarding chemistry and I am more than happy to discuss with you.

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AP Chemistry Question of the Day --63

Welcome back to my blog. Following is Question 62 I posted earlier.

 

 62. Which solid reacts with dilute hydrochloric acid at 25 °C to produce a gas that is more dense than air?

The correct answer is D. This a tough question to most students because I found a lot of students studying AP know little about predicting product of reactions. 

First of all , we need to figure out which one of them will react with dilute hydrochloric acid and generate a gas. Zn is an active metal and will react with dilute acid and produce hydrogen gas. For B, C, and D, they will undergo double replacement reactions with dilute HCl and produce a salt and another acid. Both HNO3 and HBr are strong acid and stay in aqueous solution. But H2CO3 is a weak acid and it decomposes to CO2 and water. 

Now we need to figure out which one of them H2 or CO2, is denser than air?

According to Avogadro's Law, at the same temperature and pressure, any gas has a same volume. While density is defined as mass per volume, the higher the molecular weight, the denser it is, since their molar volume is the same. Air has an average molecular weight of 29, H2 is 2 and CO2 is 44. Thus CO2 is denser than air, D is the correct answer.

Following is question 63 of the series:

63.  A 20.00 mL sample of a Ba(OH)2  solution is titrated with 0.245 M HCl. If 27.15 mL of HCl is required, what is the molarity of the Ba(OH)2  solution?
(A)  0.166 M
(B)  0.180 M

(C)  0.333 M
(D)  0.666 M

Thank you very much for reading my blog. I am available to discuss any questions regarding all levels of chemistry. Please give me a call at 604-808-6827 if you need any assistance.

AP Chemistry Question of the Day --62

Welcome back to my blog. Following is Question 61 I posted earlier.

 61. In an experiment to determine the percentage of water in a solid hydrate by heating, what is the
best indication that all the water has been removed?
(A)  The solid melts.
(B)  The solid changes color.
(C)  Water vapor no longer appears.
(D)  Successive weighings give the same mass.

The correct answer is D. The water in a  hydrate can be removed by heating. As the water is removed, the mass of the sample will decrease. If successive weighings give the same mass, this means all the water has gone. Depending on what size of your sample is, you may not be able to observe water vapour. As a result, C is not correct even though it looks reasonable. Some of the solid will change colour while some do not, B is not always true. A is not correct because it is not related to water removing. When the solid melt, it is at its melting point and that is all you know. 

Following is question 62 of the series:

 62. Which solid reacts with dilute hydrochloric acid at 25 °C to produce a gas that is more dense than air?

Thank you very much for reading my blog. I am available to discuss any questions regarding all levels of chemistry. Please give me a call at 604-808-6827 if you need any assistance.

2014 AP Chemistry Score released

The 2014 AP Chemistry score has been released. Overall it is harder to get a 5 than in 2013.
Here is the stat on the average 2014 scores:

AP Score	Total Points Required	Percentage of AP Chemistry Examinees Who Achieved These Points
5	72-100	10.1%
4	58-71	16.6%
3	42-57	25.7%
2	27-41	25.8%
1	0-26	21.8%

Here is a comparison between the score distribution between 2013 and 2014.

AP Score	2013	2014
5	18.9%	10.1%
4	21.5%	16.6%
3	18.8%	25.7%
2	14.9%	25.8%
1	26.0%	21.8%

In general, it is harder to get a high score this year. 

The major contributor to this discrepancy is the introduction of the new curriculum framework. The free response part of the exam was overwhelming and most of the students (73%) did not finish.

If you want to learn AP chemistry with me, give me a call at 604-808-6827 and I am more than happy to help you in achieving a 5 in 2015.

AP Chemistry Question of the Day --60

Welcome back to my blog. Here is question 59 I posted last week.

59.  Which of the following best helps to account for the fact that the F^- ion is smaller than the O^2- ion?                                                     

(a) F^- has a larger nuclear mass than O^2-    

(b) F^- has a larger nuclear charge than O^2-   

(c) F^- is more electronegative than O^2-        

(d) F^- is more polarizable than O^2-.

The correct answer is b.  Both fluoride and oxygen ions have 10 electrons. They are isoelectronic to each other. The size of the ions is determined by the number of protons in the nucleus. Since fluoride has 9 and oxygen has 8 protons. The electrons in fluoride ion experience more attraction from the nucleus and makes the ions smaller.

Statement A and C are both correct but they are not the right explanation.

Here is question 60 of the series:

60. Glycerol has lower vapour pressure than does ethanol at room temperature. Which of glycerol and ethanol has the lower boiling point?

Thank you very much for visiting my blog. I am available to discuss any topics in Chemistry at all levels. Please shoot me an email at vancouverchemistrytutor@gmail.com or give a call at 604-808-6827.

AP Chemistry Question of the Day --59

Welcome back to my blog. Following is question 58 of the series I posted yesterday:

58. A mixture of 64g oxygen and 66g of CO₂ are contained in a vessel exerting a total pressure of 980mmHg. Find the pressure exerted by each gas?

According to Dalton's Law of partial pressure, the pressure of a gas mixture is the sum of the partial pressures of the individual components of the gas mixture. As the mixture share the same container, the gases have the same volume and temperature. 

From the Ideal Gas Law. PV=nRT, the partial pressure of each gas is directly proportional to its mole number. If total pressure is known, the partial pressure of each gas would be the total pressure times its mole fraction.

Hope you understand this. Following is question 59 of the series:

59.  Which of the following best helps to account for the fact that the F^- ion is smaller than the O^2- ion?                                                     

(a) F^- has a larger nuclear mass than O^2-    

(b) F^- has a larger nuclear charge than O^2-   

(c) F^- is more electronegative than O^2-        

(d) F^- is more polarizable than O^2-.

I will post the answer in my next post.

Thank you very much for visiting my blog. I am available to discuss any topics in Chemistry at all levels. Please shoot me an email at vancouverchemistrytutor@gmail.com or give a call at 604-808-6827.

AP Chemistry Question of the Day --58

Welcome back to my blog. Here is question 57 I posted earlier:

The correct answer is C. This is a typical chemical equilibrium question. According to Le Chaterlier's Principle, when there is a disturbance applied to an equilibrium, the equilibrium will shift to the direction to minimize the disturbance. From the graph, it can be seen that hydronium concentration decreased. This indicate the equilibrium shifted to the left. 

Addition of HCl will increase hydronium concentration since HCl is a strong acid and it dissociate into hydronium ion and chloride ion when dissolved in water. 

Since the reaction is exothermic, decreasing the temperature will shift it towards the products. As a result, hydronium concentration will increase.

Change in volume only shifts equilibrium involving gas. As there is no gas in this equilibrium, volume will not affect the equilibrium.

Following is question 58 of the series:

58. A mixture of 64g oxygen and 66g of CO₂ are contained in a vessel exerting a total pressure of 980mmHg. Find the pressure exerted by each gas?

I will post the answer in my next post.

Thank you very much for visiting my blog. I am available to discuss any topics in Chemistry at all levels. Please shoot me an email at vancouverchemistrytutor@gmail.com or give a call at 604-808-6827.