1999 AP Chem Questions

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Discus: SAT/ACT Tests and Test Preparation: May 2004 Archive: 1999 AP Chem Questions
By Rogerfederer (Rogerfederer) on Monday, May 10, 2004 - 01:35 am: Edit

Had some questions I couldn't solve...so here goes

31) the T of an aqueous solution of NaCl is increased from 20 to 90degress Celcius. Which of the following is true?
(a) density of solution remains unchanged
(b) molarity of solution remains unchanged
(c) molality of solution remains unchaged
(d) mole fraction of solute decreases
(e) mole fraction of solute increases

54) Which of the following changes alone would cause a decrease in the value of Keq for the reaction below?

2NO(g) + O2(g) <-> 2NO2(g) delta H is negative
(a) Decrease in T
(b) increase in T
(c) Decreasing the volume of the reaction vessel
(d) increasing the volume of rxn vessel
(e) adding a catalyst

60) NH4NO3(s) -> N2O(g) + 2H2O(g)
a .003mol sample of NH4NO3(s) decomposes completely according to the balanced equation above. The total pressure in the flask measured at 400K is closest to which of the following? (The value of the gas constant, R, is .082)
(a) 3 atm
(b) 1 atm
(c) .5atm
(d) .1atm
(e) .03atm

65) Which of the following compounds is not soluable in water but is soluable in dilute hydrochloric acid?
(a) Mg(OH)2 (s)
(b) (NH4)2CO3 (s)
(c) CuSO4 (s)
(d) (NH4)2SO4 (s)
(e) Sr(NO3)2 (s)

67) What is the molar solbility of Ag2CrO4 in water? (The Ksp for Ag2CrO4 is 8*10^(-12) )
(a) 8*10^(-12)
(b) 2*10^(-12)
(c) (4*10^(-12))^(1/2)
(d) (4*10^(-12))^(1/3)
(e) (2*10^(-12))^(1/3)

here are the answers
E, B, A, A, E.

By Rogerfederer (Rogerfederer) on Monday, May 10, 2004 - 01:44 am: Edit

by the way, I saw the explanations for 31 and 54. I'll give up on 31, but for 54, wouldn't increasing the volume cause the equilibrium to shift to the left, making the Keq lower?

By Pinacolada (Pinacolada) on Monday, May 10, 2004 - 08:49 am: Edit

31. molality doesn't change with temperature. that's why it's used in freezing point depression and boiling point elevation. it's only based on mass so if the volume of the solution changes bc it evaporates at high temp etc. then the molality is still constant

54. K is only changed by temperature. if you change the volume/concentrations you end up with different concentrations of products and reactants at equilibrium but the same equilibrium constant.
the temp change is kind of like le chateliers principle - think of heat as a product since it's exothermic. to decrease Keq, you want more reactants so you add heat to the product side.

60. hmm i think it's .03 mole sample instead of .003? i used P=nRT/V and n = (.03) + 2(.03) and i set the volume to 1L. not sure about this one but it would come out to 2.95

65. Mg(OH)2 is affected by HCl because the H+ will bond to the OH- to make water and Mg(OH)2 dissociates more to replace the OH- ions. in general weak bases are more soluble when you add strong acids, and weak acids are more soluble when you add strong bases

67. in the equilibrium chart you would set up, Ag changes by +2x and cro4 changes by +x. Ag is squared because of the 2. ksp = (2x)^2 * (x) = 4x^3. solve for x

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