A 1.00-g solid sample containing a mixture of table salt (NaCl) and citric acid (H3C6H5O7, a triprotic acid) is dissolved in 15 mL of water. Titration of the acid solution requires 40.00 mL of 0.2003 M NaOH solution to reach the endpoint. Calculate the mass percent H3C6H5O7 in the solid mixture. Can somone plz explain how to do this problem...??