Potassium hydroxide is very soluble in water, resulting in extremely basic solutions. A 121g sample KOH is dissolved in water at 25∘C to make up 100.0mL of solution. The molar mass of KOH is 56.11gmol. What is the pH of the solution at 25.0∘C?

15.33

Explanation:

Given parameters:

Mass of KOH = 121g

Volume of solution  = 100mL   = 0.1dm³

Molar mass of KOH  = 56.11g/mol

Unknown:

pH  of the solution = ?

Solution:

To find the pH, we must first know the concentration of the solution obtained by mixing KOH up to this volume.

This can be done by finding the molarity of the solution.

  Molarity  =

  Number of moles of KOH  =    =   = 2.16mole

Input parameters;

 Molarity of solution =   = 21.6moldm⁻³

       KOH    →    OH⁻   +          K⁺

       21.6           21.6              21.6

In the solution we have 21.6moldm⁻³ of OH⁻ which is need to find the pH;

          pOH  = -log₁₀(OH⁻)

         pOH  =  -log₁₀21.6

          pOH  = -1.33

Since pH + pOH   = 14

          pH  = 14 - (-1.33)  = 15.33

the answer is d. 74.02