Calculate the molar mass of a diprotic acid.

It reaches the endpoint of the titration with NaOH at 28ml.
The initial concentration of NaOH is 0.09792. and the initial mass of the diprotic acid is 0.685g. I keep getting a molar mass of 500g/mol. but the options are all around 100-150.

2 answers

molesbase=1/2 moles acid
.028*.09792*1/2 .685/molmass

molmassacid= .685/(2* .028*.097920

I don't get anything near 500g/mol
So you need to times the molarity by two?