A carboxyl group is inductively electron withdrawing.  As we have already shown, the presence of electron withdrawing groups can be expected to increase the acidity of compounds.  Hence we might expect that compounds with two acid groups would be stronger acids, and this is indeed the case as illustrated by the following values:

Carboxylic Acid pKa Dicarboxylic derivative pKa Dicarboxylic derivative
Methanoic 3.8 Oxalic 1.2
Ethanoic 4.8 Malonic 2.8
Propanoic 4.9 Succinic 4.2

Maleic and Fumaric Acids

Maleic acid is a much stronger acid than fumaric, even though they are simply the cis and trans isomers of butenedioic acid:

Acid Structure pKa
Maleic 1.9
Fumaric 3.0

This is due to intramolecular hydrogen bonding, which can occur with maleic, but not fumaric acid:

This causes the cis anion to be more stable than the trans.

Note, however, that this extra stabilisation must be ovecome for a second deprotonation to occur. Thus the second deprotonation occurs more readily for fumaric acid than maleic.