What is the pH of the mixture?

Question

Room temperature sample of water with a volume of 230.1 mL is placed into a flask. A volume of 2.3 mL of 0.0351 M NaOH is then added to the water in the flask. 

Answer 1

To find the pH, we need to know the concentration of hydrogen ions (H+). Since we have a base, NaOH, we can first find the [OH-] and then find [H+], or we can find the pOH and then the pH. I'll do it both ways.

moles OH- present = 2.3 ml x 1 L/1000 ml x 0.0351 mol/L = 8.07x10^-5 mols

Final volume = 230.1 ml + 2.3 ml = 232.4 ml = 0.2324 L

Final [OH-] = 8.07x10^-5 mol / 0.2324 L = 3.47x10^-4 M

pOH = -log 3.47x10^-4

pOH = 3.46

pH = 14 - 3.46

pH = 10.5

[H+][OH-] = 1x10^-14

[H+] = 1x10^-14 / 3.47x10^-4

[H+] = 2.88x10^-11

pH = -log 2.88x10^-11

pH = 10.5