AP Chemistry Question of the Day --64

I have not written here for a long time. AP season is coming soon in a couple of months. I will resume my series and hopefully it is helpful to anyone who come across to visit this site. Instead of posting a question and answer it the next day, I am going to post the question and answer at the same time.

Question 64: Which one of the following cidic solutions requires the most 0.1000M NaOH aqueous solution to neutralize completely:
A. 0.1000M HCl 25ml
B. 0.1000M HF 25ml
C. 0.05000M H2SO4 25ml
D. 0.05000M H2CO3 25ml
E. They are all the same

The answer to this question is E. When an acid and a base react, whenever there is a strong acid or base involved, the reaction will go to completion. The amount of base required will be totally determined by the mole number of the protons in the acid. It does not matter the acid is strong or weak, monoprtotic or diprotic. It is the number of moles of H+ that is in the acid that determines how much base will be needed to completely neutralize.

Hope this helps with your understanding on acid and base and thank you for visiting my blog.

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 --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.