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F325 Ka help

Hi, I'm trying to revise for OCR F325 paper desperately after the f324 disaster and every time I do a Ka question like the one attached (ii) I always calculate the moles of the weak acid wrong for some reason and I can't figure out why. The mark scheme says the moles of butanoic acid are 0.0100 but I always get 0.0125 when I work it out.
Is there something I'm doing wrong? Could anyone help please?
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A_pratt
OP
The question is from the June 2012 EEE paper, it's "The student adds 50.0 cm3 0.250 moldm-3 butanoic acid to 50.0 cm3 0.500 moldm3 sodium hydroxide. Calculate the PH of the buffer solution if the Ka of butanoic acid is 1.51x10-5 moldm3"
(edited 8 years ago)
Original post by A_pratt
Hi, I'm trying to revise for OCR F325 paper desperately after the f324 disaster and every time I do a Ka question like the one attached (ii) I always calculate the moles of the weak acid wrong for some reason and I can't figure out why. The mark scheme says the moles of butanoic acid are 0.0100 but I always get 0.0125 when I work it out.
Is there something I'm doing wrong? Could anyone help please?
Attachment not found


So the initial moles of butanoic acid is 0.0125 moles like you said. However, it reacts with the sodium hydroxide to form 0.0025 moles of the butanoate ions. This leaves 0.0125-0.0025 moles of butanoic acid -> 0.01 moles

Best way to do question like this is to write out the equations happening:

C3H7COOH + OH- -> C3H7COO- + H2O

C3H7COOH <-> C3H7COO- + H+

and label moles of each thing (you label moles of the complete reaction first and then put those values into the equation where the weak acid dissociates.)
(edited 8 years ago)
Reply 3
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A_pratt
OP
Thank you! That makes so much more sense now!

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