What is the pH of a solution prepared by dissolving 0.40 g of NaOH (molar mass 40 g/mol) in water to make 250 mL of solution? Assume complete dissociation.
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Correct answer
D. 12.60
Principle or equation
pH = -log[H+]; for a strong base, pOH = -log[OH-] and pH + pOH = 14 at 25°C. Concentration is amount of substance divided by volume in litres.
Why this answer is correct
First calculate moles of NaOH: 0.40 g / 40 g/mol = 0.010 mol. Then concentration = 0.010 mol / 0.250 L = 0.040 mol/L. Since NaOH dissociates completely, [OH-] = 0.040 M. pOH = -log(0.040) = 1.40. Therefore pH = 14 - 1.40 = 12.60.
Example
For 0.020 mol NaOH in 500 mL, [OH-] = 0.040 M, pOH = 1.40, pH = 12.60.
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