Strong base is dissolved in 595 mL of 0.200 M weak acid (Ka = 3.34 × 10-5) to make a buffer with a pH of 3.99. Assume that the volume remains constant when the base is added.Calculate the pKa value of the acid and determine the number of moles of acid initially present. How many moles of strong base were initially added?When the reaction is complete, what is the concentration ratio of conjugate base to acid?

Question

anonymous · Answer

So for the first question I got pka is 4.48 and there is .119 mol HA. and the concentration ratio is .32 but I cant seem to find how many moles of strong base were initially added? i tried many ways but I keep getting it wrong.

anonymous · Answer

concentration ratio is correct... i got .326 by doing

pH-pKa 
OR 
Ka/[H+]

see attached picture for the icf and ice tables. i used NaOH to represent the strong base and HF to represent weak acid.

here's the final setup $$.326 = \frac{ (\frac{n} {.595}+1.02e-4) }{ (\frac{ .119-n }{ .595 }-1.02e-4) }$$

$$n=.292$$ moles of strong base added

anonymous · Answer

i meant: 

10^(pH-pKa) for the ratio between base and acid -- my mistake.
or
ka = [base][H+]/[acid] --> divide [H+] on both sides --> ka/[H+]=[base]/[acid]