A 0.315 g sample of Tums (containing CO2−3CO32- ) is dissolved in 25.0 mL of 0.102 M HCl

Question

How many mol of base are in the antacid?

The hydrochloric acid that is not neutralized by the Tums is back titrated with 8.75 mL of 0.102 M NaOH

Answer 1

Tums is calcium carbonate, CaCO3.  Not sure what you mean by CO2-3CO2-.  I'll asssume it means CaCO3 or CO3^2- (carbonate).

moles HCl added = 0.025 L x 0.102 mol/L = 0.00255 moles HCl

moles HCl left after reaction with tums = 0.00875 L x 0.102 mol/L = 0.0008925 moles HCl (from NaOH+HCl =>H2O+NaCl 1:1 mol ratio)

moles HCl used in titration = 0.00255 - 0.0008925 = 0.001658 moles

2HCl + CO3^2- ==> H2CO3 ==> CO2 + H2O

moles CO3^2- present = 0.001658 mol HCl x 2 mol HCl/mol CO3^2- = 0.00332 moles base present in sample.