Answer
Citric
Work Step by Step
Number of moles of the base:
$0.513\ M\times 29.1\ mL=14.93\ mmol$
If it's tartaric, number of moles:
$0.956\ g\div 150.09\ g/mol=6.37\ mmol$
Ratio: $2.34$
If it's citric, number of moles:
$0.956\ g\div 192.12\ g/mol=4.98\ mmol$
Ratio: $3.00$
Since the ratio is closer to the stoichiometric coefficient, it's citric acid.
