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PH buffer calculations

I'm doing buffers and PH in chemistry at the moment.
I understand how to work out the PH from the buffer solution but i was wondering if anyone could help with this question:

What mass of sodium methanoate should be dissolved in 250cm3 of 0.100m methanoic acid to form a buffer solution with a PH of 5.2
(ka for methanoic acid= 1.78x10^-4 moldm3)

Thanks

Reply 1

Borek

K_a=\frac {[HCOO^-][H^+]} {[HCOOH]}

K_a - given
[H⁺] - given (well, pH is given, but that's just a single operation)
[HCOOH] - given

Solve for the only unknown, calculate what mass of HCOONa is needed to get this concentration of methanoate ions.

Reply 2

cuckoo99

Original post by amy.louise

(someone correct me if I'm wrong i may be a bit rusty)

-Rearrange the Ka expression to find [HCOO-] where [HCOO-] is the concentration of methanoate ions

(you can find [H+] by doing the inverse log of the ph)

-Find moles of sodium methanoate using an equation that links concentration and volume
-Find mass by using an equation that links moles and Mr