Calculate the pH of each of the following solutions:

Question

a. [H3O+ ] = 2.0 x 10^-12 M

b. [OH- ] = 4.5 x 10^-3 M

c. 3.5 M HCl

d. 5.25 M NaOH

Answer 1

pH = -log [H₃O⁺]

pH + pOH = 14

[H₃O⁺][OH^-] = 1x10^-14

a) pH = -log 2.0x10^-12 = 11.7 = pH

b) pOH = -log 4.5x10-3 = 2.35 and pH = 14 - 2.35 = 11.7 = pH

c) 3.5 M HCl = 3.5 M H3O+ and pH = -log [H3O+] = -log 3.5 = -0.5 = pH

d) 5.25 M NaOH = 5.25 M OH- and pOH = -log [OH-] = -0.72; pH = 14 - pOH = 14 - (-0.72) = 14.72 = pH

Note: the values presented are not rounded to the correct number of significant figures, as you can do that yourself. Recall that sig figs when taking logs are determined by the number of decimal places in the value of which you are taking the log.