A solution of acetic acid is prepared in water by adding 11.1 g of sodium acetate to 200 ml of water. The final ph is measured to be 6.5. What are the concentrations of acetate and acetic acid in solution?

Question

Esteban Lynch

Chemistry

17 January, 06:45

A solution of acetic acid is prepared in water by adding 11.1 g of sodium acetate to 200 ml of water. The final ph is measured to be 6.5. What are the concentrations of acetate and acetic acid in solution?

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Danny Mcbride · Answer 1

Given buffer contains:

Acetic acid: CH3COOH.

Sodium Acetate : CH3COONa.

Mass of CH3COONa taken = 11.1 g

Molar mass of CH3COONa = 82.03 g/mol

# moles of CH3COONa = 11.1g/82.03 g. mol-1 = 0.1353 moles

Volume of water = 200 ml = 0.2 L

Therefore, the initial concentration of CH3COONa = 0.1353/0.2 = 0.677 M

Buffer pH = 6.5

pKa (CH3COOH) = 4.76

Based on Henderson-Hasselbalch equation:

pH = pKa + log [CH3COONa]/[CH3COOH]

Now, initially there is only 0.677 M CH3COONa. If 'x' is the concentration of the protonated species (i. e CH3COOH) formed, then the amount of CH3cooNa remaining = 0.677-x

6.5 = 4.76 + log [0.677-x]/[x]

x = 0.011 M

i. e. [CH3COOH] = 0.011 m

[CH3COONa] = 0.677-0.011 = 0.666 M