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Acid-base equilibria question

at.midnight

How do you find the pH for this question? I’ve highlighted the answer. Thank you.

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Reply 1

CJField

for b, remember Kw = [OH][H]... you can figure out ph quite easily

for d, find moles of acid after reaction, then remember that Ka = [H]^2 / [HA]

Reply 2

at.midnight

@CJField could you please explain how to do d) step by step cause I really don’t get how to do it? Thanks for answering!!

Reply 3

CJField

Original post by at.midnight

@CJField could you please explain how to do d) step by step cause I really don’t get how to do it? Thanks for answering!!

Note - I assume your familiar with the notation of HA, H and A, as well as square brackets for conc. If not just say.

Okay so you need to figure out how much acid is left. Calculate the initial moles of acid and the moles of NaOH added, then minus the moles of NaOH from the moles of acid to find the new moles of acid. You can then find out [HA].

Next we need to use Ka=[H][A] / [HA]
Because [H] = [A], we can use Ka = [H]^2 / [HA]
Note, because only a very small amount of HA disassociates - weak acid - we don't need to include the decrease of [HA] from disassociation in our calculation.

Rearrange the above equation for [H], then use pH = -log[H]