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How to Determine the pH of 0.1 Mol NaAc

NaAc abbreviates the sodium salt of acetic acid with the chemical formula CH3COOH. Sodium acetate undergoes the hydrolysis according to the following equation: CH3COO(-) + Na(+) + H2O = CH3COOH + Na(+) + OH(-). Hydroxide ions (OH-) released in this reaction make the solution more basic increasing its pH. PH quantitatively measures the acidity of solutions, and is the negative logarithm of the concentration of hydrogen ions (H+): pH=-log (H+). To calculate the concentration of hydrogen ions take into the account the product of the concentrations of ions H+ and OH(-) is constant and equal to the ionization constant of water.

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Instructions

- 1
  Divide the ionization constant of water (10^-14) by the dissociation constant of acetic acid to calculate the hydrolysis constant. The dissociation constant of this acid is 1.77 x 10^-5 and hence the hydrolysis constant is 10^-14/ 1.77 x 10^-5 = 5.65 x 10^-10.
- 2
  Write down the equation that describes equilibrium concentrations of molecules in the reaction of hydrolysis:
  
  [OH-] x [CH3COOH] / [CH3COO-] = 5.65 x 10^-10 (Hydrolysis constant)
  
  Note that concentrations are denoted with the square brackets.
- 3
  Denote the unknown concentration of hydroxide ions (OH-) as "X" to convert the equlibrium concentration equation to the mathematical form noting also that the concentrations of [OH-] and [CH3COOH] are equal to each other as follows from the dissociation chemical equation:
  
  X^2 / 0.1 = 5.65 x 10^-10
  
  The given concentration of acetate CH3COO- is 0.1.
- 4
  Multiply both sides of the mathematical equation by 0.1 as follows:
  
  (X^2 / 0.1) x 0,1 = (5.65 x 10^-10) x 0.1 or
  
  X^2 = 5.65 x 10^-11.
- 5
  Take the square root of the number 5.65 x 10^-11 to calculate the concentration of hydroxide ions:
  
  [OH-] = square root (5.65 x 10^-11) = 7.5 x 10^-6.
- 6
  Divide the ionization constant of water (10^-14) by the concentration of hydroxide ions to calculate the concentration of hydrogen ions (H+) in the solution:
  
  10^(-14) / 7.5 x 10^-6 = 1.3 x 10^(-9).
- 7
  Take the logarithm of the concentration of hydrogen ions, and then multiply it by -1 to calculate the pH. The pH of the NaAc solution is (-1) x log (1.3 x 10^-9) = -1 x (-8.9) = 8.9.

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