6. A titration was carried out to determine the concentration of 12.5 mL of an HCl(aq) solution. 9.0 mL of 0.050 mol/L barium hydroxide solution was required to reach the endpoint. Calculate the concentration of chloride ions in the reaction mixture at the end of the titration.
2HCl + Ba(OH)2 ==> BaCl2 + 2H2O
For such small numbers I like to work in millimoles. That is a way to keep all those zeros.
mols Ba(OH)2 = M x L = 0.05 x 0.009 = 0.00045 but
millimols Ba(OH)2 = M x mL = 0.05 x 9 = 0.45
Convert mmols Ba(OH)2 to mmols HCl. From the equation 2 mol HCl = 1 mol Ba(OH)2; therefore, 2 x 0.045 = 0.9 millimols HCl
M HCl = M/L = mmols/mL = 0.9/12.5 = 0.072 M.
(HCl) = 0.072 so (Cl^-) must be 0.072 M also since there is 1 atom Cl per 1 molecule of HCl.