A- level chemistry acid base questions

Hi, I'm stuck on these questions I am able to work out the moles for each of the acids but then I don't know what to do and I can't seem to get the right pH.

Calculate the pH of the solution formed when 100 cm3
of 0.100 mol dm-3 NaOH is added to 20 cm3
of 0.150 mol dm3 H2SO4.

Calculate the pH of the solution formed when 50 cm3
of 0.100 mol dm-3 NaOH is added to 100 cm3
of 0.300 mol dm3 CH3COOH (Ka = 1.74 x 10-5 mol dm-3).

Calculate the pH of the solution formed when 50 cm3
of 0.0500 mol dm-3 Ba(OH)2 is added to 20 cm3
of 0.100 mol dm-3 HCOOH (Ka = 1.78 x 10-4 mol dm-3).

Reply 1

Sodium229

Original post by PPP170903

These questions are of similar ideas, for each question, try to work out whether the acid or the base is in excess, and then you calculate the excess moles of acid or base. If the base is in excess, you calculate the concentration of the excess base by dividing the number of excess moles of the base by the total total volume of the solution and then you need to use the fact that Kw=[OH-][H+] and Kw=1.00 x 10^-14, rearrange to find [H+] and then find the pH by using the formula pH=-log[H+]; if the acid is in excess, then calculate the concentration of the excess acid by dividing the number of excess moles of the acid by the total volume of the solution, then use the formula that pH = -log[H+] to get your answer. Be careful though, diprotic acid such as H2SO4 in the first question means that 1 mole of H2SO4 gives 2 moles of H+, similarly for the dibasic base like Ba(OH)2 in the third question means that 1 mole of Ba(OH)2 gives 2 moles of OH-. Hope this helps :smile: