Question

a. The pH of an aqueous solution of 0.272 M potassium hypochlorite, KClO (aq), is: __________

is this solution acidic/basic/neutral?

b. The pH of an aqueous solution of 0.274 M sodium cyanide, NaCN (aq), is: __________

is this solution acidic/basic/neutral?

Answer 1

a. The pH of an aqueous solution of 0.272 M potassium hypochlorite, KClO (aq), is: 10.445

is this solution acidic/basic/neutral? : basic

b. The pH of an aqueous solution of 0.274 M sodium cyanide, NaCN (aq), is: 11.416

is this solution acidic/basic/neutral? : basic

Explanation

(a.) initial concentration of ClO^- = initial concentration of KClO = 0.272 M

K_a HClO = 3.5 x 10^-8

K_b ClO^- = (K_w) / (K_a)

K_b ClO^- = (1.0 x 10^-14) / (3.5 x 10^-8)

K_b ClO^- = 2.9 x 10^-7

ICE table	ClO- (aq)	H2O (l)	$\rightleftharpoons$	HClO (aq)	OH- (aq)
Initial conc.	0.272 M	-		0	0
Change	-x	-		+x	+x
Equilibrium conc.	0.272 M - x	-		+x	+x

K_b = [HClO]_eq[OH^-]_eq / [ClO^-]_eq

2.9 x 10^-7 = [(x) * (x)] / (0.272 M - x)

Solving for x, x = 2.786 x 10^-4 M

[OH^-] = x = 2.786 x 10^-4 M

pOH = -log[OH^-]

pOH = -log(2.786 x 10^-4 M)

pOH = 3.555

pH = 14 - pOH

pH = 14 - 3.555

pH = 10.445

(b.) initial concentration of CN^- = initial concentration of NaCN = 0.274 M

K_a HCN = 4.0 x 10^-10

K_b CN^- = (K_w) / (K_a)

K_b CN^- = (1.0 x 10^-14) / (4.0 x 10^-10)

K_b CN^- = 2.5 x 10^-5

ICE table	CN- (aq)	H2O (l)	$\rightleftharpoons$	HCN (aq)	OH- (aq)
Initial conc.	0.274 M	-		0	0
Change	-x	-		+x	+x
Equilibrium conc.	0.274 M - x	-		+x	+x

K_b = [HCN]_eq[OH^-]_eq / [CN^-]_eq

2.5 x 10^-5 = [(x) * (x)] / (0.274 M - x)

Solving for x, x = 2.786 x 10^-4 M

[OH^-] = x = 2.6 x 10^-3 M

pOH = -log[OH^-]

pOH = -log(2.6 x 10^-3 M)

pOH = 2.584

pH = 14 - pOH

pH = 14 - 2.584

pH = 11.416