Explanation:
The given data is as follows.
        Mass of mixture = 0.3471 g
As the mixture contains oxalic acid and benzoic acid. So, oxalic acid will have two protons and benzoic acid has one proton.
This means oxalic acid will react with 2 moles of NaOH and benzoic acid will react with 1 mole of NaOH.
Hence,   moles of NaOH in 97 ml = 
                                                        = 
Moles of HCl in 21.00 ml = 
                                          =  mol
 mol
Therefore, total moles of NaOH that reacted are as follows.
             -
 -  
       
                 =  mol
 mol
So, total 3 mole of NaOH will react with 1 mole of mixture. Therefore, number of moles of NaOH reacted with benzoic acid is as follows.
                   
                     =  mol
 mol
Since, molar mass of NaOH is 40 g/mol. Therefore, calculate the mass of NaOH as follows.
                     
                          =  g
 g
                          = 0.0832 g
Whereas molar mass of benzoic acid is 122 g/mol.
Therefore,       40 g NaOH = 122 g benzoic acid
So,            0.0832 g NaOH = 
                                              = 0.253 g
Hence, calculate the % mass of benzoic acid as follows.
                       
                            = 73.10%
Thus, we can conclude that mass % of benzoic acid is 73.10%.