Question

Consider a buffer solution that is 0.50 M in NH3 and 0.20 M in NH4Cl. For ammonia, pKb=4.75. Calculate the pH of 1.0 L of the solution upon addition of 30.0 mL of 1.0 M HCl to the original buffer solution.

Express your answer to two decimal places.
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Answer 1

You have 0.50 mol of NH3 and 0.20 mol of NH4+ to start (NH4Cl dissolves completely), given the molarity and 1.0 L solution.

30.0 mL of 1.0 M HCl is 0.0300 mol of HCl. This will react with the NH3 to produced 0.030 mol of NH4+.

You now have 0.47 mol NH3 and 0.23 mol NH4+. Now use the Henderson-Hasselbach equation to calculate your pH. The equation says to use concentration of acid and base, but you can just use the moles of them because it doesn’t make a difference.

pH = pKa + log(base/acid)

pKa = 14 - pKb = 14 - 4.75 = 9.25

pH = 9.25 + log(0.47/0.23) = 9.56

Answer 2

The pH of the solution is 9.56.

Given:

Mol of NH₃ = 0.50

Mol of $NH_4^+$ = 0.20 to start (NH₄Cl dissolves completely)

Molarity = 1.0 M

30.0 mL of 1.0 M HCl is 0.0300 mol of HCl.

This will react with the NH₃ to produced 0.030 mol of $NH_4^+$ .

Henderson-Hasselbach equation:

This equation is used to calculate pH. It is an approximate equation that shows the relationship between the pH or pOH of a solution and the pKa or pKb and the ratio of the concentrations of the dissociated chemical species

pH = pKa + log(base/acid)

pKa = 14 - pKb

pKa= 14 - 4.75

pKa= 9.25

pH = 9.25 + log(0.47/0.23)

pH = 9.56

Thus, the pH of the solution is 9.56.

Find more information about pH here:

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