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The Acid Dissociation Constant & pH: Past Paper Question Analysis

P3 June 2017 Q4

This question is about pH and the acid dissociation constant

If you haven't gone through the paper, please look up the questions at www.aqa.org.uk, or click the direct link below, and have a go...

This question is in 6 parts with 9 marks available

Question 4

Look at the top and bottom of the vertical part of the curve. The change in pH will be very rapid indicating a small drop of sodium hydroxide.

The range looks like 7 - 10 - you don't want to extend the range to include the parts where the curve veers away from the vertical.

The mark is given for any two values within the range 7 - 10.2

The expression is in the form Ka = [dissociated ions]/[acid]

We know the concentration from part 3. We just need to rearrange the acid dissociation constant...

Notice that it is assumed that the concentration of the ions will be equal to each other - which is why they are replaced by the concentration of hydrogen ions squared.

The concentration of HX will not be much different than that calculated in part 3 since it is a weak acid and only slightly dissociates.

Make sure you can recall the equations for the dissociation constant and pH. Quote pH to 2 decimal places

Recall that the pH at half-neutralisation = pKa.

The marks are for...

Both points are plotted correctly and the line touches both points

Line steeper at the start then levels (to show buffering)

(Some responses from students assumed that half-equivalence was at 10 or 15 cubic cm and so their plots were incorrect).

Please share your thoughts through the comments box below - I look forward to hearing from you.


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