Question

An unknown contains citric acid, C6H8O7, a tribasic acid, and potassium chloride. If 2.147g of unknown requires 20.10mL of 0.678M NaOH for complete neutralization of all reactive protons, calculate the percent by mass of citric acid in the sample. Indicate clearly your calculations and reasoning.

Answer 1

NaOH:

moles = Molarity * Volume

          = 0.678*(20.10/1000)

          = 0.0136 moles

C6H8O7(aq) + 3NaOH(aq) ----> Na3C6H5O7(aq) + H2O(l)

3 moles of NaOH requires -----> 1 mole of C6H8O7

0.0136 moles of NaOH requires -----> (1*0.0136)/3

                                                      = 0.00453 moles of C6H8O7

mass of C6H8O7 = moles* molarmass       ( citric acid molar mass = 192 g/mol )

                          = 0.00453*192

                          = 0.8698 g

percent by mass of citric acid in the sample = (0.8698/2.147)*(100)

                                                                      = 40.51%