A Level Chemistry question - acid base titration pH calulation

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HydraFly
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I have done the first part of this following question:

20cm3 of methanoic acid (Ka = 1.8x10^-4 moldm^-3) of concentration 0.10 moldm^-3 is titrated against NaOH of concentration 0.05 moldm^-3

Calculate the pH of the solution:
- Initially (I worked this out to be 2.37)
- After 10cm3 of the alkali is added
- After 20cm3 is added
- After 30cm3 is added
- After 50cm3 is added

How do I do this?? I get the first one...
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<3<3
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After the alkali is added, this reacts with the acid to form salt and water. Therefore a buffer is formed containing the acid and conjugate salt and therefore the buffer equation can be used to find [H+].

Step 1. Find the number of moles of alkali that are added and the number of moles of acid currently in the solution
2. Take away the smallest from the largest to find the moles of salt formed and the moles of acid left.
3. Find new concentrations of the salt and acid by dividing each by the total volume in dm^3
4. Plug into equation: to find [H+]
Ka=[H+][salt]
[acid]
5. Find pH by -log[H+]
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