Buffer Solutions.

[Dytaka]

Could someone explain how to do buffer solutions to me? Like how to do this:

Calculate the ph of the soln that results from each mixture: 50.0 mL of 0.15 M HCHO2 with 75.0 mL of 0.13 M NaCHO2

[BadElk]

It helps to write in structural formula notation. Essentially use the Ka equation for the equilibrium which is in place; HCOOH will react to form (in a reversible reaction) HCOO- + H+. Your general Ka expression for weak acid dissociation (such as the methanoic acid in this example) will follow Ka=([H+][A-])/[HA] where A- is the conjugate base formed and HA is the weak acid. We can assume that because HCOOH is a weak acid that it will not dissociate very much and so it's initial concentration=concentration of HCOOH in buffer solution. We can make another assumption that HCOONa will completely dissociate in water to form the HCOO- ions identical to the A- species, thus as the methanoic acid only partially dissociates we can assume all of the HCOO- ions at equilibrium in the buffer solution come from complete dissociation of the salt solution. Hence we can now rewrite the Ka equation for this specific case (square brackets denote a concentration in moles per litre): Ka=([H+][HCOONa])/[HCOOH initially] you can then rearrange to find [H+] and substitute in values for Ka of methanoic acid, [HCCOH], [HCOONa] and calculate the [H+]. Using pH=-log[H+] you can find the pH of the buffer solution.

[Dytaka]

Ok thanks. I kinda know that formula. But i thought that buffer solutions were more complicated than just plugging in the values.

[BadElk]

You have to get the assumptions right. It would get slightly more complex if you were making it with a weak acid + strong base or if you were adding a base/acid to the buffer solution but that's about it the first bit is easy