A 100 mL solution of .500 M hydrocyanic acid is titrated with a .125 M solution of potassium hydroxide?

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2018-04-09 9:11 am

What is the pH of the solution after 500 mL of the potassium hydroxide solution has been added? Ka for hydrocyanic acid is 4.9 x 10-10. pKa of hydrocyanic acid is 9.31

回答 (1)

wanszeto

2018-04-09 10:08 am

✔ 最佳答案

Before titration:
Initial no. of moles of HCN = (0.500 mol/L) × (100/1000 mL) = 0.0500 mol
Initial no. of moles of OH⁻ = (0.125 mol/L) × (500/1000 mL) = 0.0625 mol
Final volume of the solution = (100 + 500) mL = 600 mL = 0.600 L

Equation for the reaction taking place in titration:
HCN + OH⁻ → CN⁻ + H₂O
Mole ratio HCN : OH⁻ = 1 : 1

OH⁻ is in excess.
[OH⁻] in the final solution = (0.0625 - 0.0500) / 0.600 M = 0.0208 M
pOH of the final solution = -log[OH⁻] = -log(0.0208) = 1.68
pH of the final solution = pKw - pOH = 14.00 - 1.68 = 12.32

(The given values of Ka and pKa for HCN are redundant.)

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