What mass of KOH would need to be dissolved in 500.0 mL of water to produce a solution with a pH of 12.40?

Answer 1

KOH is a strong base and will exist in an aqueous solution as ions. Its ionic equation is:

KOH (aq) → K⁺ (aq) + OH^- (aq)

1) Calculate the pOH from the pH.

pH + pOH = 14.00

pOH = 14.00 - 12.40 = 1.60

2) Calculate the [OH^-] from the pOH.

[OH-] = 10^-1.60 = 0.025 M [OH^-]

3) Use volume and Molarity to calculate moles of OH^-.

M = n / V

V is in mL. Need to convert to L first.

V = 500.0 mL x (1 L / 1000 mL) = 0.5000 L

n = M x V = 0.025 mol/L x 0.5000 L = 0.0125 mol OH-

4) Convert mol OH^- to g using the MW of KOH.

g KOH = 0.0125 mol x 56.11 g/mol = 0.70 g KOH