Question

Determine the pH of the following solutions:

a: 50.0mg of naproxen sodium dissolved in 1.00mL water

b: 1.0x10^-3M (1mM) cocaine

c: 1.00g cocaine HCL dissolved in 100.0mL water

Answer 1

a. concentration of naproxen sodium in water = 0.050 g/252.23 g/mol x 0.001 ml = 0.2 M

sodium salt hydrolyses to give OH- and naproxen in solution, with x amount hydrolyzed

Kb = 1.41 x 10^-10 = x^2/0.2

x = [OH-] = 5.31 x 10^-6 M

pOH = -log[OH-] = 5.27

pH = 14 - pOH = 8.73

b. cocaine is a weak base with takes up a proton from water to form salt and OH- in solution

let x amount has reacted

Kb = 2.64 x 10^-6 = x^2/1 x 10^-6

x = [OH-] = 1.625 x 10^-6 M

pOH = 5.79

pH = 14 - pOH = 8.21

c. concentration of cocaine HCl = 1.0 g/339.814 g/mol x 0.1 L = 0.03 M

hydrolyses in water to form H3O+ and cocaine

Ka = 3.80 x 10^-9 = x^2/0.03

x = [H+] = 1.07 x 10^-5 M

pH = 4.97