Question

Hello, I have a quick question regarding an experiment. The goal is to find the molar mass/identity of the unknown acid that I titrated with 0.09758M NaOH solution. The unknown acid is a diprotic weak acid. The pH at equivalence point 1 is 4.62 at 20.00mL and pH at the second equivalence point is at 8.56 at 40.00mL. The molar mass of the acid that was dissolved in 50ml of DI water was 0.2531g. Also, help me identify the unknown acid. Could you please show your work on how you got the molar mass so I can understand the process. Thanks! :)

Answer 1

1) H2A + 2 NaOH ----> Na2A + 2H2O(l)

   H2A = unknown diprotic acid

   1 mol H2A = 2 mol NaOH

no of mol of NaOH reacted = v*M
  
                           = 40*0.09758/1000

              = 0.003903 mol

no of mol of H2A reacted = 0.003903/2 = 0.0019515 mol

Molarmass of unknown acid = w/n = 0.2531/0.0019515

    = 129.7 g/mol