A solution of acetic acid is prepared in water by adding 11.1 g of sodium acetate to a volumetric flask and bringing the volume to 1.0 L with water. The final pH is measured to be 5.25. What are the concentrations of acetate and acetic acid in solution? (Assume that the temperature of the solution is 25 degrees Celsius)

Molarity acetate: 0.1353M

Molarity acetic acid: 0.0418M

Explanation:

When in solution you have a weak acid (Acetic acid) and its conjugate base (sodium acetate) a buffer is produced. The pH of this buffer is obtained using the H-H equation:

pH = pKa + log [A⁻] / [HA]

Where pH is pH of the solution = 5.25

pKa is pKa of acetic buffer = 4.74

[A⁻] is molarity of acetate ion - Molar mass sodium acetate: 82.03g/mol-:

11.1g * (1mol / 82.03g) = 0.1353moles A⁻ / 1.0L = 0.1353M

And [HA] are moles of acetic acid.

Solving for HA:

5.25 = 4.74 + log [0.1353] / [HA]

0.51 =  log [0.1353] / [HA]

3.236 = [0.1353] / [HA]

[HA] = 0.1353M / 3.236

[HA] = 0.0418M

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