Thursday, November 15, 2018

CHEM 121 and CHEM 233 tutor

Final exam is coming soon, if you need help with your CHEM 121 or CHEM 233, do not hesitate to contact me at 604-808-6827. No matter you goal is to get a higher A or just to pass, I can help you accomplish it.

Friday, September 7, 2018

UBC Chemistry Tutor

September again. I am an experienced chemistry tutor. I can help you with your UBC CHEM 121, CHEM 123, CHEM 233, CHEM 205, CHEM 203 and CHEM 213. No matter your goal is to pass or ace to get into medical school, I can help you. If you need a tutor, give a call at 604-808-6827.

Tuesday, March 27, 2018

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.

Thursday, March 15, 2018

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

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Monday, May 15, 2017

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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Thursday, May 4, 2017

UBC chemistry tutor

The new UBC summer term is coming very soon. If you need help with CHEM 121/123. CHEM 233/205, please call me at 604-808-6827 and I am more than happy to help you meet your goals.

Friday, September 2, 2016

Dr. Zhang Chemistry Tutoring 张老师化学家教

Ph. D in Chemistry with years of experience offering chemistry tutoring on AP Chemistry, IB Chemistry, SAT Chemistry, Chemistry 11 and 12, University Level General Chemistry and second year Organic Chemistry including UBC CHEM 111, CHME 113, CHEM 121, CHEM123, CHEM 154, CHEM 203, CHEM 213, CHEM 233, CHEM 205, CHEM 235 and CHEM 245, Langara College CHEM 1114, CHEM 1118, CHEM 1120 and CHEM 1220. I can teach with both English and Mandarin. Please call 604-808-6827. 南开大学化学系博士,UBC化学系博士后,多年经验辅导AP化学,IB化学,SAT化学,十一年级和十二年级化学,大学一年级普通化学和二年级有机化学和物理化学,因材施教,深入浅出,中英文结合,帮你理解要点,抓住重点,取得满意成绩。请电604-808-6827.