Question

Aspirin (180.15 g/mol) has a pKa of 3.5. What is the pH of a solution of aspirin of a normal adult dose (650mg) in an 8 oz. glass of water?

Answer 1

volume of water = 8 oz = 0.2366 L

mass of aspirin = 650 mg = 0.650 g

moles of aspirin = 0.650 / 180.15

                         = 3.608 x 10^-3 mol

concentration = 3.608 x 10^-3 / 0.2366

                       = 0.01525 M

pKa = 3.5

Ka = 3.16 x 10^-4 M

HA ---------------> H+ + A-

0.01525                  0        0

0.01525 - x             x        x

Ka = x^2 /0.01525 - x

3.16 x 10^-4 = x^2 /0.01525 - x

x = 2.04 x 10^-3

[H+] = 2.04 x 10^-3 M

pH = -log (2.04 x 10^-3)

pH = 2.69