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 --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.

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 --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.