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

Welcome back to my blog. Since most schools are doing the unit on acid and base at this time of the year and here is another question related to this topic.

Question 65: Four acids HA, HB and HC and HD solutions have the same pH, which of the following statements is correct:
A. They all have the same concentration
B. They are all strong acids
C. They are all weak acid
D. not enough information to make any conclusion

The correct answer is D. pH represents the concentration of hydronium ion in solution. The pH if an acidic solution is dependent on both the Ka and the concentration of the acid. A solution of a strong acid can have a higher pH due to lower concentration and a solution of a weak acid cna have a lower pH due to higher concentration.

Hope this helpful.

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AP Chemistry Question of the Day --61

Welcome back to my blog. here is Question 60 I posted earlier.
60. Glycerol has lower vapour pressure than does ethanol at room temperature. Which of glycerol and ethanol has the lower boiling point?

The correct answer should be ethanol.

Vapour pressure is the pressure of the vapour when at a certain temperature the liquid and the vapour reach equilibrium in a closed container. Vapour pressure is only dependent on the nature of the substance and temperature. For a given substance, its vapour pressure is only dependent on temperature.  As temperature increases, vapour pressure increases. When the vapour pressure reaches the atmosphere pressure, the temperature is defined as boiling point. As a result, the higher the vapour pressure, the lower the boiling point.

Following is question 61 of the series:

 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.

I will post the answer in my next blog.
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 --57

Welcome back to my blog. Here is question 56 I posted earlier:

The correct answer is B. From the potential diagram, it can be concluded that the products have lower potential energy than the reactants and the reaction is exothermic. According to La Chatellier's Principle, the equilibrium will shift to endothermic side when temperature is increased. The endothermic direction is the reverse of the above direction. Since equilibrium shifts to left, the K will decrease.

Following is question 57 of the series:
57.

I will post the answer in my next blog.

Thank you very much for reading my post, I am available to discuss any topics in 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.

Regards!

AP Chemistry Question of the Day --56

Welcome back to my blog. Here is question 55 I posted earlier. 
55. Aqueous solutions of AgNO3, Cu(NO3)2 and Cr(NO3)3 are electrolyzed using the same quantity of electricity. How do the number of moles of metal formed compare?

A.      Ag = Cu = Cr

B.      Ag > Cu > Cr

C.      Ag < Cu < Cr

D.      Cu > Ag > Cr

The correct answer is B. To answer this question, we need to understand how electrolysis works. In an aqueous electrolytic cell, redox reaction happens so that the cation of the salt or hydrogen ion from water is reduced to its elemental form and the anion or hydroxide from water is oxidized to its oxidation product or oxygen. Whether water will be involved depends on the reduction potentials of the cation and the anion. If the anion has a lower reduction potential than hydroxide, then its oxidation is dominant. If the cation has a higher reduction potential than hydrogen, its reduction is then dominant. Also we need to take into account of the pH and over potential effect of water. In this question, since all of the metal ions have higher reduction potential than hydrogen, they are going to be reduced. As diver ion, cooper ion and chromium ion has a charge of +1, +2 and +3 respectively, to get one mole of metal reduced, they will need 1, 2 and 3 moles of electrons respectively. If the same amount of electricity is consumed, then the mole number of the metals would be: 1mole Ag:0.5mole Cu:0.33mole chromium. Thus, B is the correct answer.

Following is question 56 of the series:

56. 

I will post the answer in my next blog.

Thank you very much for reading my post, I am available to discuss any topics in 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.

Regards!

AP Chemistry Question of the Day --55

Welcome back to my  blog. Here is question 54 I posted yesterday. The correct answer is B.

There are several factors affecting reaction rate such as temperature, concentration, surface area and catalyst. But here what is being tested is the collision theory.
For a chemical reaction to happen, the reactant molecules have to collide with enough energy and proper orientation. If the reactants molecules are easier to bump into each other, the reaction should occur relatively faster. As a result, from the four choices above, B is the correct answer. A and D are reactions in gas phase. Gas molecules are far apart and are not likely to collide as easily as in solution. C is a heterogeneous reaction. Again the reaction is limited at the surface of the lead.

Following is Question 55 of the series,

55. Aqueous solutions of AgNO3, Cu(NO3)2 and Cr(NO3)3 are electrolyzed using the same quantity of electricity. How do the number of moles of metal formed compare?

A.      Ag = Cu = Cr

B.      Ag > Cu > Cr

C.      Ag < Cu < Cr

D.      Cu > Ag > Cr

I will post the answer in my next blog.

Thank you very much for reading my post, I am available to discuss any topics in 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.

Regards!

张老师化学家教

Experienced Ph. D. in chemistry offering tutoring service for AP Chemistry, IB Chemistry, SAT II Chemistry, Chemistry 11 and Chemistry 12.  Personalized tutoring tailored to individual needs. Guaranteed improvement and success. 

I can tutor with both English and Chinese.

For one on one tutoring needs, please call 604-808-6827.

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