Question

You wish to prepare an hc2h3o2 buffer with a ph of 4.34. if the pka of is 4.74, what ratio of c2h3o2/hc2h3o2 must you use?

Answer 1

From Henderson-Hasselbalch equation:

PH = Pka + ㏒[conjugate base / weak acid]

when C2H3O2 is the conjugate base 

and HC2H3O2 is the weak acid

so, 
PH = Pka + ㏒[C2H3O2]/[HC2H3O2]

by substitution:

4.34 = 4.74 + ㏒[C2H3O2][HC2H3O2]

∴[C2H3O2]/[HC2H3O2] = 0.4