Question

how many moles of hydrogen are evolved when an excess of sodium metal is added to one mole of citric acid?

Answer 1

The formula of citric acid is:

           CO2H
           |
HO -  C - CH2CO2H
           |
          C - CH2 CO2H



I bolded the 4 hydrogens that can be subsituted by sodium.


Three of those hydrogens belong to carboxyl group (- CO2H) and one to a hydroxyl group (- OH).


You need to take into account that 4 H will form 2 H2 molecules.


So, the answer is that 2 moles of molecules will be formed from 1 mole of citric acid molecules.