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A2: Titration curve Q

Hi, I'm not if lack of sleep has turned my mind to mush but I'm a bit confused regarding this question:

sketch the titration curve for this reaction when 30 cm³ of the sodium hydroxide is added to 25.0 cm³ of the vinegar solution.

I understand all the pH points: starts at 3, equivalence range is ~5-11, ends at 13. But I don't understand why the mark scheme says:
"Sigmoid curve starting between pH 2 and 4
(2.8), ending between pH 12 and 14 inclusive (1)
with steep rise (may be vertical or gently
sloping) of between 3 – 7 units between pH 6
and 12. Sloping section should not extend over
more than 5cm3 (1)
When 12.5 cm3 NaOH added. (1)"

I thought the equivalence point would be at 25cm3 when there are equal amounts of acid and base, then it would extend to 13 at 30cm3. How come it's at 12.5cm3? Thank you!

Reply 1

ThisIsOurDecision

Original post by LeaX

I thought the equivalence point would be at 25cm3 when there are equal amounts of acid and base, then it would extend to 13 at 30cm3. How come it's at 12.5cm3? Thank you!

It is because the NaOH solution is more concentrated than the vinegar solution. If the concentration of OH^- and H⁺ was equal, then yes the equivalence would be at 25cm³ added.

Reply 2

Borek

Not clear if the question states anything about concentrations.