This question appeared in the FARSIDE team's first packet for VETO 2006.

BONUS 6
[[ HAND OUT PAGE WITH GRAPH ]]
You've just been handed a page with a titration curve showing the pH of a solution of a weak monoprotic acid against volume of sodium hydroxide solution added to it. For 10 points per answer:

A. Within zero point two millilitres, how many millilitres of acid solution were present initially?

Answer:  12 mL (accept in range 11.8 — 12.2 mL)
(read from centre of vertical jump)

B. Within a factor of two, what is the acid dissociation constant of the acid?

Answer:  10^-6 (ten to -6 power)   (accept in range 5 x 10^-7 — 2 x 10^-6)
(raise 10 to the minus power of pH at half-equivalence point, which is where there is half as much NaOh as acid solution — that is, 6 mL of NaOH)

C. If the concentration of acid solution is equal to the concentration of the sodium hydroxide solution, then what is this concentration, within a factor of two?

Answer:  0.01 M   (accept in range 0.005 to 0.02 M)
(from initial pH of 4, get [H⁺] = 10^-4 M; square this and divide by K_a = 10^-6)

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