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pH calculation

This question concerns the titration of a solution of sodium hydroxide with a solution of
hydrochloric acid. As the titration proceeds the pH of the mixture changes.
(a) What was the pH when 24.95 cm3 of 1.00 mol dm–3 NaOH(aq) had been added to
25 cm3 of 1.00 mol dm–3 HCl(aq)?
A 3
B 6
C 8
D 11

(b) What was the pH when 25.05 cm3 of 1.00 mol dm–3 NaOH(aq) had been added to
25 cm3 of 1.00 mol dm–3 HCl(aq)?
A 3
B 6
C 8
D 11

i can calculate part (a) but i cant calculate (b) by using the same method.
someone please help!
Do you know how to calculate the pH of alkaline solutions?

The first question is essentially, calculate the pH of a strong acid, the second is the pH of a strong base.
Reply 2
Avatar for Killjoy-
Killjoy-
8
With part a) you end up with excess protons in solution after the reaction has taken place.
But with part b) [OH-]>[H+] so you end up with hydroxide ions in solution after the reaction has taken place.

Does that help?

You need to find the hydroxide concentration and then use Kw=[H+(aq)][OH−(aq)]K_w =[\mathrm{H^{+}(aq)}][\mathrm{OH^{-}(aq)}] or 14=pOH + pH.
Reply 3
Avatar for 041087
041087
OP
2
Original post by Killjoy-
With part a) you end up with excess protons in solution after the reaction has taken place.
But with part b) [OH-]>[H+] so you end up with hydroxide ions in solution after the reaction has taken place.

Does that help?

You need to find the hydroxide concentration and then use Kw=[H+(aq)][OH−(aq)]K_w =[\mathrm{H^{+}(aq)}][\mathrm{OH^{-}(aq)}] or 14=pOH + pH.


oh, i got it. thz:smile:
i have one more question to ask,which say

This question concerns four solutions, A to D. They were prepared by mixing equal
volumes of 0.2 mol dm–3 solutions of two different substances. The substances were
A HCl(aq) and NaOH(aq)
B HCl(aq) and NaCl(aq)
C NH3(aq) and NH4Cl(aq)
D CH3COOH(aq) and CH3CO2Na(aq)
Select, from A to D, the mixture which would have a chloride ion concentration of 0.2 mol dm–3.
(edited 11 years ago)
Original post by 041087
oh, i got it. thz:smile:
i have one more question to ask,which say

This question concerns four solutions, A to D. They were prepared by mixing equal
volumes of 0.2 mol dm–3 solutions of two different substances. The substances were
A HCl(aq) and NaOH(aq)
B HCl(aq) and NaCl(aq)
C NH3(aq) and NH4Cl(aq)
D CH3COOH(aq) and CH3CO2Na(aq)
Select, from A to D, the mixture which would have a chloride ion concentration of 0.2 mol dm–3.


Write out a balanced equation for each reaction and work out the stoichiometric ratios :smile:

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