Hello! I've got some Chemistry homework but I think I've done one of the questions wrong.
What mass of sodium methanoate should be dissolved in 250cm3 of 0.100M methanoic acid to form a buffer solution with a pH of 5.20? (ka of methanoic acid = 1.74x10^-4.)
[H+] = 10^-5.2 = 6.31x10^-6
[H+] = ka x [HA]/[A-]
6.31x10^-6 = 1.78x10^-4 = 0.1/[A-]
[A-] = 2.82 mol dm^-3
moles = vol x conc
moles of A- = 2.82 x 0.25 = 0.705
This is where I get confused.
Are the moles of A- the same as the moles of HCOONa???
What I did:
I worked out the Mr of HCOONa to be 68.
mass = moles x Mr
mass = 0.705 x 68 = 47.94
final answer = 47.9 g.
I'm pretty sure I did the last part wrong. To summarise, my actual question is: for a weak acid, are the moles of [A-] the same as the moles of [HA]?
Sorry for the confusing post. Thanks!
Is this right?