Question

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?

Answer 1

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.

Answer 2

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

Answer 3

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]

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