CHEM II PRACTICE EXAM IV ANSWERS

DISCLAIMER: ASSUME THAT THE X APPROXIMATION IS VALID FOR ALL THE FOLLOWING PROBLEMS.

1. A solution made up of 0.2 M HBr and 0.1 M NaBr can be considered a buffer: a.True b.False

2. a. List the characteristics of a buffer. Resist ph change Weak acid/base with its conjugate base/acid respectively The acid/base neutralizes the added base/acid respectively b. Define the following terms: equivalence point: point at which moles of acid and bases in solution cancel out. end point: point during the titration, at which there is little to no change in ph (the indicator does not change color)indicator: usually a dye, that changes based on the ph level. reversible reaction: written with the equilibrium arrows. Infinitesimally small changes can guide the reaction forward or reverse. common ion effect: presence of a common ion that causes the acid or base to ionize even less than normal. Always drives the reaction to the left. spontaneous process: Happens without external intervention (energy, work) entropy: Determines the direction of a spontaneous change. Tendency for randomness of molecules, energy etc…

3. Calculate the pH of the buffer that is 0.255 M in CH3NH2 and 0.135 M in CH3NH3Br Kb=4.4*10-4 pH=10.92

4. In which of these solutions will HNO3 ionize less than it does in pure water?a. 0.1 M NaCL b. 0.1 M NaNO2

c. 0.1 M KNO3

d. 0.1 M NaOH

5. Calculate the ratio of CH3NH2 to CH3NH3Cl concentration required to create a buffer of pH=10.24Kb=4.4*10-4 . RATIO IS 0.398

6. A 1.0 L buffer solution is 0.125 M in HNO2 and 0.145 M in NaNO2. Determine the concentrations of HNO2 and NaNO2 after the addition of each substance.a. 1.5 g HCl 0.104 mol NO2 left. 0.166 mol HNO2

b. 1.5 g NaOH 0.0875 mol HNO2 left. 0.1825 mol NaNO2

c. 1.5 g HI 0.133 mol NO2 left. 0.137 mol HNO2

7. Consider the titration of a 35.0 ml sample of 0.175 M HBr with 0.200 M KOH. Determine each quantity.a. The initial pH

Since we have a strong acid, we can calculate the pH by using the concentration of the strong acid.pH=-log (0.175)= 0.757

b. The volume of added base required to reach the equivalence point (0.03L of KOH)c. The pH at 10.0 ml of added base (pH=1.04) d. The pH at equivalence point (pH=7)e. The pH after adding 5.0 ml of base beyond the equivalence point (pH=12.09)

8. Predict the signs of entropy changes for the following physical/chemical processes or chemical reactions:a. Deposition of ice on a windshield (-)b. Melting of snow above 0 oC (+)c. The evaporation of ethanol at room temperature. (+)d. 6 CO2 (g) + 6 H2O(g) C6H12O6 (s) + 6 O2 (g) (-)e. HCl (g) + NH3 (g) NH4Cl (s) (-)f. Mg(s) + Cl2 (g) MgCl2 (s) (-)

9. Which of the following processes are nonspontaneous? a. A bike going up a hill b. A meteor falling to Earthc. Obtaining hydrogen gas from liquid waterd. A ball rolling down a hill e. Combustion of natural gasf. A hot drink cooling to room temperature

10. Consider the titration of a 25.0 ml sample of 0.175 M CH3NH2 with 0.150 M HBr. Determine each quantity. Kb=4.4*10^-4a. The initial pH ((Disregard the fact that the x approximation is a little over 5%, on your exam

you should only have examples where the x approximation is valid)After using the ICE table, find pOH then pH=11.94)

b. The volume of added acid required to reach the equivalence point (Volume=29.16 ml of HBr)

c. The pH at 5.0 ml of added acid (Once you find the number of moles of the base and it’s conjugate acid, don’t forget to divide by the total volume since the added acid solution and the initial base solution don’t have the same molarity)

d. The pH at one-half of the equivalence point pH=10.64

e. The pH at the equivalence point

f. The pH after adding 5.0 ml of acid beyond the equivalence point

11. For each group of molecules, determine which one has the highest standard molar entropy and why? a) NO (g) and N2O4 (g) molecular complexityb) NaCl (aq) and NaCl (s) Dissolution

c) Graphite because as an allotrope it has more room for movement then a diamond structure does.

d) Na (s) and K(s) Higher molar masse) H2 (g) and CaCO3 (s) Gas state

12- Calculate the ΔSorxn for the following reaction: (USE APPENDIX IIB)

a. 3 NO2 (g) + H2O (l) 2 HNO3(aq) + NO (g) ΔSorxn= -45.7 j/mol*K

b. SO2 (g) + (1/2) O2 (g) SO3 (g) ΔSorxn=-94 j/mol*K

13- T OR F

a. Buffer capacity is unaffected by changes in absolute concentrations of the buffer components F

b. The more concentrated the weak acid and conjugate base that make up the buffer, the higher the buffer capacity T

c. Overall buffer capacity decreases as the relative concentrations of the buffer components become more similar to each other F

d. As the ratio of the buffer components gets closer to 1, the overall capacity of the buffer increases. T

14- A 500 ml buffer solution is 0.1 M in HNO2 and 0.150 M in KNO2. Determine if each addition would exceed the capacity of the buffer to neutralize it.

a. 250 mg NaOH Doesn’t exceed capacityb. 1.25 g HBr Doesn’t exceed capacity

15- Match types of titrations with the titration curves below:

1) Strong acid-strong base (A)2) Strong base-strong acid (D)3) Weak base-strong acid (C) 4) Weak acid-strong base (E)5) Polyprotic acid-strong base (B)

A ) B) C)

D) E)

16- What is the pH at half-equivalence point in the titration of a weak base with a strong acid? The pkb of the weak base is 7.63

Ph=6.37