Chemistry 142

George fall’08

Lec Quiz - Acid/base buffers name

SHOW ALL WORK FOR CREDIT

1. (20pts) Solution A is prepared by dissolving 32.0 g of solid NH₄NO₃ (80.0 g/mole) in 500 mL of 0.50 M NH₃. (Assume the solution volume is still 500 mL.)

(a) SHOW ALL WORK. Determine the pH of solution A.

Straight equilibrium_

NH₄⁺_(aq) + H₂O_(l) ßà NH_3(aq) + H₃O⁺_(aq)

I (mol) 0.40 ~ 0 0.25 ~0

∆ - x -- +x + x

E 0.40-x -- 0.25 + x x

(0.25 + x)(x) = 10^-14 approx x to “0” and solve

0.40 –x 1.8 x 10^-5

X = 8.9 x 10^{-10 and} pH = 9.0 (1 sig fig)

(b) SHOW ALL WORK. Suppose that 100 mL of 2.50 M NaOH is added to Solution A. Determine the pH of the resulting solution.

1^st do the chemistry that goes to completion

OH^-_{(aq) +} NH₄⁺_(aq) à HOH_(l) + NH_3(aq)

Mol before rxn 0.25 0.40 0.25

Mol used rxn -0.25 -0.25 +0.25

Mol after rxn “0” 0.15 0.50

2^nd now for the equilibrium

NH_3(aq) + HOH_(l) ß à NH₄⁺_(aq) + OH^-_(aq)

I 0.50 -- 0.15 “0”

Δ -x -- +x +x

E 0.50 – x o.15 +x +x

Now plug into K_b for NH₃ and approximate x to zero-

(0.15 mol NH₄⁺/0.60L)(x) = 1.8 x 10^-5, giving x = 6.0 x 10^-5, and pH = 9.8

(0.50 mol NH₃/0.60L)

Note_the volumes cancel and there is 1 sf

(c) SHOW ALL WORK. Suppose instead that 100 mL of 2.50 M HNO₃ is added to Solution A. Determine the pH of the resulting solution.

1^st do the chemistry that goes to completion

H⁺_(aq) + NH_3(aq) à HOH_(l) + NH₄⁺_(aq)

Mol before rxn 0.25 0.25 -- 0.40

Mol used rxn -0.25 -0.25 +0.25

Mol after rxn “0” “0” 0.65

2^nd now for the equilibrium

NH₄⁺_(aq) + H₂O_(l) ßà NH_3(aq) + H₃O⁺_(aq)

I 0.65 -- “0” “0”

Δ -x -- +x +x

E 0.65 – x +x +x

Use K_a which is K_w/ K_b = 1.0 x 10^-14 = x²

1.8 x 10^-5 (0.65/ mol/0.60L)

-logx = pH = 4.6