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Buffers question

Stuck on this buffer question...
Q4eiii) from the f325(EEE) June 15 paper. Student adds 6.075g Mg to 1.00dm^3 of the buffer solution, calculate new Ph of buffer?
Previous info: Buffer solution contains propanoic acid, C2H5COOH and propanoate ions. The concentrations are both 1.00 mol dm-3. Ka for C2H5COOH is 1.35*10^-3.
Any help appreciated :smile:
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Original post by a.d.3006
Stuck on this buffer question...
Q4eiii) from the f325(EEE) June 15 paper. Student adds 6.075g Mg to 1.00dm^3 of the buffer solution, calculate new Ph of buffer?
Previous info: Buffer solution contains propanoic acid, C2H5COOH and propanoate ions. The concentrations are both 1.00 mol dm-3. Ka for C2H5COOH is 1.35*10^-3.
Any help appreciated :smile:

I'm thinking the Mg reacts with the propanoic acid to produce propanoate. So you have 1mole each of acid and propanoate to start, as it is 1moldm-3 and you have 1dm3 vol.
Work out moles of Mg. Moles of acid decreases by number of moles of Mg, and moles of propanoate increase by moles of Mg. You now have new moles of each present, which is the same ratio as concentration. Rearrange Ka expression to give H+. Sub in your values to work out [H+] and then convert to pH.

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