[kershaw22]

A 75 mL solution that is 0.10 M in acetic acid and 0.10 M in sodium aceate has a pH of 4.74. Which of the following actions will change the pH of this solution?
a) Adding 15 mL of 0.10 M HCL
b) Adding 0.010 mole of sodium acetate
c) Diluting the solution from 75 mL to 125 mL

Why do a and b work, but not c?

Thanks

[Borek]

Have you heard about buffer solutions?

[sjb]

A 75 mL solution that is 0.10 M in acetic acid and 0.10 M in sodium acetate has a pH of 4.74. Which of the following actions will change the pH of this solution?
a) Adding 15 mL of 0.10 M HCL
b) Adding 0.010 mole of sodium acetate
c) Diluting the solution from 75 mL to 125 mL

Why do a and b work, but not c?

Thanks

At a simple level, how are you calculating the pH of the initial solution?

[Nicho55]

A 75 mL solution that is 0.10 M in acetic acid and 0.10 M in sodium aceate has a pH of 4.74. Which of the following actions will change the pH of this solution?
a) Adding 15 mL of 0.10 M HCL
b) Adding 0.010 mole of sodium acetate
c) Diluting the solution from 75 mL to 125 mL

Why do a and b work, but not c?

Thanks

Diluting the solution does not affect the concentration of H⁺ ions, which determine the pH of the solution: pH= -log₁₀([H⁺]) assuming it is diluted with deionised or distilled water.

[Nicho55]

Haha, scratch that last post, of course what I put makes no sense!

[kershaw22]

I'm aware that the HCL can dissociate into H+ and Cl-, so the H+ would lower the pH, and that the sodium acetate can dissociate into Na+ and C2H3O2-, which would raise the pH slightly. According to the question, there is no effect on pH from diluting, but I was wondering why.

[AWK]

I'm aware that the HCL can dissociate into H+ and Cl-, so the H+ would lower the pH, and that the sodium acetate can dissociate into Na+ and C2H3O2-, which would raise the pH slightly. According to the question, there is no effect on pH from diluting, but I was wondering why.

This statement is true within Henderson-Hasselbach approximation. Of course a real change of pH of order 0.01 to 0.02 pH unit in this case is negligible small. In the cases a) and b) the changes of pH are  ~20 times greater.
For a) case new concentrations of a weak acid and a salt should be calculated assuming stronger acid replaces a weaker one completely.
For b) case concentration of salt (Bronsted base) should be recalculated.