Calculating the pH of a weak acid titrated with a strong b. An analytical chemis

Question

Calculating the pH of a weak acid titrated with a strong b. An analytical chemist is titrating 220.3 mL of a 0.7900 M solution of propionic acid (HC,H,CO2) with a 0.7100 M solution of NaOH. ThepK, of propionic acid is 4.89. Calculate the pH of the acid solution after the chemist has added 83.62 mL of the NaOH solution to it. Note for advanced students: you may assume the final volume equals the initial volume of the solution plus the volume of NaOH solution added. Round your answer to 2 decimal places. 2

Explanation / Answer

millimoles of acid = 220.3 x 0.7900 = 174.04

millimoles of NaOH added = 83.62 x 0.7100 = 59.37

174.04 - 59.37 = 114.67 millimoles acid left

59.37 millimoles salt formed

total volume = 220.3 + 83.62 = 303.92 mL

[acid] = 114.67 / 303.92 = 0.377 M

[salt] = 59.37 / 303.92 = 0.195 M

pH = pKa + log [salt] / [acid]

pH = 4.89 + log [0.195] / [0.377]

pH = 4.60

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