this should really be in chemistry forum mate but still here goes.....
a) log ratio is just pH-pKa=-1.41
b) to get ratio
log (S - 0.004306 / 0.004306)=-1.41
10^log (S - 0.004306 / 0.004306)=10^-1.41
10^log (base 10) =1 so
(S - 0.004306 / 0.004306)=ratio= 10^-1.41
c) rearrange and solve for S
i.e. since i have no calculator, let 10^-1.41=x
S= (0.004306x)/0.004306
edit: i have probably done this wrong as i rushed it
i think u have to make it
(S - 0.004306 / 0.004306)=(S/0.004306)-1
and so ratio is (S/0.004306)
woopsi!
sorry but at least the method is right