ATAR Notes: Forum

VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: deledio on October 30, 2008, 01:44:50 pm

Title: Buffer Question
Post by: deledio on October 30, 2008, 01:44:50 pm

q: A mixture of 0.040M Na2HPO4 and 0.040 KH2PO4 acts as a buffer, maintaining pH at approx. 7. Use equations to show how the mixture can resist a change when a small amount of acid or base is added.

a:...

Title: Re: Buffer Question
Post by: bturville on October 30, 2008, 01:56:45 pm

Write up equilibrium arrows between the two equations, showing that adding more H+ results in more of the Na2HPO4 being produced, and when base is added the Na2HPO4 loses a proton returning to KH2PO4.

Title: Re: Buffer Question
Post by: Pandemonium on October 30, 2008, 02:01:40 pm

Na2HPO4(aq) forms HPO4 2-(aq) which is a weak base.
KH2PO4(aq) forms H2PO4 -(aq) which is amphoteric.

So, when you add a little H3O+:
HPO4 2-(aq) + H3O+(aq) <-> HPO4-(aq) + H2O(l)
H2PO4-(aq) + H3O+(aq) <-> H3PO4(aq) + H2O(l)

Also, when you add a little OH-:
HPO4 2-(aq) + OH-(aq) <-> PO4 3-(aq) + H2O(l)
H2PO4-(aq) + OH-(aq) <-> HPO4(aq) + H2O(l)