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help!!! past paper question chemistry

(6CH04) June 2012 https://qualifications.pearson.com/content/dam/pdf/A%20Level/Chemistry/2013/Exam%20materials/6CH04_01_que_20120613.pdf
Q14 IV....
I've been struggling with this question......
I know I'm dumb
would you guys mind helping me
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Pigster
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Original post by universalcj
Q14 IV....


Imagine you had 1 mol of a weak acid, dissolved into 1 dm3 of solution. I assume, if you were given Ka you could work out the pH (you'll make the assumption that [H+] = [A-] and use square root)

Now, if you added 0.1 mol of NaOH(s), then hopefully you agree that it will react with 0.1 mol of HA leaving 0.9 mol of HA unreacted (which can then dissociate as per Ka), but 0.1 mol of A- will form.

Now, your earlier assumption is no longer valid [H+] =/= [A-], i.e. you have yourself a buffer solution. BUT, you know Ka (unchanged) and you know [HA] (0.9) and you know [A-] (0.1), so you can work out [H+] and hence pH.

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