Calculate the pH of a solution made by mixing 20.0 mL of a 0.100M methanoic acid (CHOOH) with 80.0 mL of a 0.05M sodium methanoate (CHOONa). Ka for methanoic acid 1.78 x 10-4.

Answer 1

When mixing a weak acid (CHOOH) with the salt of that acid (CHOONa), you are making a buffer.

First, we'll calculate the concentration of each substance:

mols CHOOH = 20.0 ml x 1 L / 1000 ml x 0.100 mol/L = 0.002 mols CHOOH

mols CHOONa = 80.0 ml x 1 L / 1000 ml x 0.05 mol / L = 0.004 mol CHOONa

Final volume = 20 ml + 80 ml = 100 ml = 0.100 L

[CHOOH] = 0.002 mol / 0.100 L = 0.02 M

[CHOONa] = 0.004 mol / 0.100 L = 0.04 M

We can now use the Henderson Hasselbalch equation to find the pH

pH = pKa + log [CHOONa] / [CHOOH]

pKa = -log Ka = -log 1.78x10^-4 = 3.75

pH = 3.75 - log (0.04/0.02)

pH = 3.75 - 0.30

pH = 3.45