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Acids and Bases - Calculating pH of a Strong Base

Worked Chemistry Problems

By Todd Helmenstine

Sodium hydroxide is an example of a strong base.

Sodium hydroxide is an example of a strong base.

Ben Mills
Question

What is the pH of a 0.05 M solution of Potassium Hydroxide?

Solution

Potassium Hydroxide or KOH, is a strong base and will dissociate completely in water to K+ and OH-. For every mole of KOH, there will be 1 mole of OH-, so the concentration of OH- will be the same as the concentration of KOH. Therefore, [OH-] = 0.05 M.

Since the concentration of OH- is known, the pOH value is more useful. pOH is calculated by the formula

pOH = - log [OH-]

Enter the concentration found before

pOH = - log (0.05)
pOH = -(-1.3)
pOH = 1.3

The value for pH is needed and the relationship between pH and pOH is given by

pH + pOH = 14

pH = 14 - pOH
pH = 14 - 1.3
pH = 12.7

Answer

The pH of a 0.05 M solution of Potassium Hydroxide is 12.7.

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