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 --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!

AP Chemistry Question of the Day --53

Welcome back to my blog. Here is question 52 I posted yesterday.

52. An 11.1L sample of 0.25mol oxygen gas is converted to ozone. If all the oxygen is converted to ozone, what will the volume of ozone be?

This is a typical stoichiometry question. To solve the question, we have to correctly write the chemical equation and properly balance it.  Ozone and oxygen are allotropes (do you still remember what is allotropes? Allotropes are different forms that an element can exist in) of oxygen and their formula are O3 and O2 respectively.

The balanced chemical reaction should be:

According to the chemical equation, 3mol of oxygen gas will produce 2mol of ozone gas. as a result, 0.25mol of oxygen should produce:

What is the volume of the ozone gas then? Recall Avogadro's law which states that  at constant temperature and pressure, the volume is directly proportional to the mole number. Since 0.25mol oxygen occupies 11.1L. apparently it is not STP. Or else, it should be approximately 0.5mol roughly 11.2L.  We know for ideal gas, the identity of the gas does not matter. As long as they have the same mole numbers, at the same T and P, the volume would be same. As a result, the volume of the ozone would be:

Pay attention that I kept 2 sig figures for my answer because the mole number of ozone has 2 sig fig.

In stoichiometric calculations, dimensional analysis is key. Make sure you align your expressions properly so that the unit can cancel out and only left with what you are asked for.

Here is Question 53 of the series:

53.  A solution appears yellow.  It must be absorbing in the  

    (a) yellow only    (b) red only    (c) green only    (d) blue only   (e) purple only. 

 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!

Langara Chemistry Tutor

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