Question

A 1.47 L buffer solution consists of 0.329 M propanoic acid and 0.188 M sodium propanoate. Calculate the pH of the solution f

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Answer #1

Solution:

According to Hendersen -Hasselbalch equation for propionic acid buffer solution,

pH = pka + log [Sodium propionate] / [Propionic acid]

pka of propionic acid is calculated as,

pka = - log ka = - log 1.34 x 10^-5

pka = 5 - log 1.34 = 5 - 0.13 = 4.87

The addition of HCl decreases the moles of conjugate base (sodium propionate) while it increases the moles of propionic acid.

Number of moles of propionic acid = Molarity x Volume

= 0.329 M x 1.47 L = 0.484 mol

Number of moles of sodium propionate = Molarity x Volume

= 0.188 M x 1.47 L = 0.276 mol

Thus,

After addition of 0.073 mol HCl,

Moles of propionic acid = 0.484 + 0.073 = 0.557 mol

Moles of sodium propionate = 0.276 - 0.073 = 0.203 mol

Therefore, pH can be calculated as,

pH = pka + log (0.203 /0.557)

pH = 4.87 + log 0.364

pH = 4.87 - 0.44

pH = 4.43

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