Chemistry Unit 4 IAL (WCH04) - June 2014

So.. how did you guys find the exam?

i found it pretty good, some questions were tricky though like the sodium lactate one; using the titration curve to suss out the ph for sodium lactate

Quite good.

Perhaps too easy?

Now I'm just worried about high grade boundaries.

does anyone remember the how to do the last question?

Does anyone know the link to Unit 5 paper. really having alot of trouble. Or can we discuss it here?

pKa of lactic acid is smaller than that of ethanoic acid, so lactic acid is a stronger acid and acts as a proton donor
so.. CH3CH(OH)COOH + CH3COOH ---> CH3CH(OH)COO- + CH3COOH2+

Hey @Biochris

remember we had to tell how the buffer is made with the help of ionic equations. I think that was the 2nd last question.. no? How did you answer that?

"First mark:
(buffer contains) reservoir of HA and A—
OR
(buffer contains) large concentrations of
[HA] and [A-]
OR
both equations:
HA ⇌ A— + H+ and NaA Na+ + A—
(1)
Second mark:
(Addition of alkali/base)
HA + OH– → A– + H2O
OR
description/equations to show that H+ reacts
with OH— (to form H2O) and more acid
dissociates (to replace H+)
(1)
Third mark:
(Addition of acid)
A– + H+ → HA
OR
A– reacting with H+ in any context described in
words (e.g. by reference to weak acid
equilibrium)
(1)
Fourth mark:
the ratio of [A–]÷[HA] hardly changes / the ratio
of [HA]÷ [A–] hardly changes
OR
[A–] nor [HA] changes significantly (1)"
This is from the Jan 2010 mark scheme