Question 6.7: Predict the pH and composition of a solution prepared by add...
Predict the pH and composition of a solution prepared by adding 0.090 mol of HF to 0.040 mol of NH_3 .
Since HF is present in excess and the reaction between HF and NH_3 is favorable, the NH_3 will react to form {NH_4}^+. At equilibrium, essentially no NH_3 remains and
Moles {NH_4}^+ = 0.040 mol
Converting NH_3 to {NH_4}^+ consumes 0.040 mol of HF; thus
Moles HF = 0.090 – 0.040 = 0.050 mol
Moles F^– = 0.040 mol
According to the ladder diagram for this system (see Figure 6.5), a pH of 3.17 results when there is an equal amount of HF and F^–. Since we have more HF than F^–, the pH will be slightly less than 3.17. Similar reasoning will show you that mixing together 0.090 mol of NH_3 and 0.040 mol of HF will result in a solution whose pH is slightly larger than 9.24.
