Use the concept of electronegativity to justify why the acid strengths of ethanedioic acid and ethanoic acid are different.

This is a long answer (6 mark) question, which means you need to demonstrate your written communication of scientific ideas.

The first thing I would do in a question is identify the key words.

Firstly, electronegativity. As soon as you see that word you should be thinking of 1) the definition; 'the power of an atom to withdraw electron density from a covalent bond', and 2) the most electronegative atoms; F, N, O and Cl.

Secondly, acid strength. You know two definitions of acidity - a Brönsted definition ('H+/proton donor), and the Lewis definition (electron pair acceptor). Unless the question specifies, it's generally easier to use the Brönsted definition.

Thirdly, the acids themselves. Always, ALWAYS draw out the species you're discussing, because it's very easy to get mixed up mid question.

I'd structure this answer by first writing down the definitions of electronegativity and acidity, as these are the concepts you'll be discussing.

Then, discuss the different structures of the acids (I know it's obvious that they're different - but the exam board wants you to state the obvious - imagine you're explaining it to someone who's never even heard of Chemistry) - they're both of the form RCOOH, with the R (R being any group) groups being different, R=CH3 in ethanoic acid, but R=COOH in ethanedioic acid. 

After that, talk about the different inductive effects of the two different R groups. The COOH R group has 2 highly electronegative oxygen atoms and so has an electon withdrawing (negative inductive) effect. On the other hand the CH3 group has a positive inductive effect, meaning it pushes electron density.

Finally, focus on the O-H bond, as this is the bond you need to break in order for the species to act as a Brönsted acid. With R=COOH the bond is more polarised, and so H is more delta positive, meaning it's easier for ethanedioic acid to dissociate in to ions than ethanoic acid. Therefore ethanedioic acid is the stronger acid.