Chem 104

Spring '02 name______________________

Quiz #3

1. citrate + HCN <== citric acid + CN^-

Will the equilibrium lie to the left or to the right? (circle it)

pKa1 for citric acid, is 3.13

pKa for hydrocyanic acid, HCN, is 9.88

The reaction goes backward because citric acid is the stronger acid.

2. Which is a stronger base, ammonia, NH3, or phenolate, C6H5O- ?

Kb for ammonia, NH3, is 1.8 x 10-5.

Ka for phenol, C6H5OH is 1.3 x 10-10

Ka for phenol, C6H5OH must be used to find the K_b:

K_w = K_a x K_b

10^-14 / 1.3 x 10^-10= Kb

7.7 x 10-5 = K_b(phenolate)

Phenolate is a stronger base since it has the larger K_b

3. c) What is the conjugate base of nitric acid? Nitrate, NO3-

d) What is the base strength of this conjugate base? Negligible since nitric acid is a strong acid.

4. 50.0 mL of perchloric acid having a concentration of 0.100 M is mixed with an aqueous solution of 0.100 g sodium hydroxide that is dissolved in 10 mL water.

(a) What are the pH and pOH of the final solution?

answersú pH = 1.38 and pOH = 12.62

0.050L HClO₄ x 0.10 M = 0.050 moles HClO₄=> 0.0050 mol H+ ; species in excess

0.100g NaOH/ 40.0 g/mol = 0.0025 mol NaOH => 0.0025 mol OH-

moles H+ left = 0.005- 0.0025 = 0.0025 mol H+

[H+] = 0.0025 mol/ (0.05L + 0.01 L) = 0.042 M => pH = 1.38

pOH = 14- pH = 12.62