Standard Solutions To make up a standard solution you usually add a liquid solution of unknown concentration to a solid that has been accurately weighted. eg. An HCl solution is standardized using pure Na2CO3(mm=106.00) as a primary standard. What is the molarity of the acid if 30.00 mL of the acid solution is required to react completely with a 0.500 gram sample of Na2CO3? Na2CO3(s) + 2 HCl(aq) ---------> 2 NaCl(aq) + H2O(l) + CO2(g) moles of Na2CO3 = 0.500 grams = 4.72 x 10-3 moles                                106.00 g/mol Reaction works on a 1:2 basis 1 Na2CO3 = 2 HCl                                                    4.72 x 10-3        x                                             x = 9.44 x 10-3 moles of HCl                                   M = moles = 9.44 x 10-3 moles = 0.315 M                                               L       0.030 L (30 mL) The molarity of the HCl is not standardized at 0.315 mole/L Follow-Up Problem A sodium hydroxide solution is standardized by reaction with benzoic acid, HC7H5O2, (mm=122.00 grams/mole). A 2.00 gram sample of benzoic acid required 35.00 mL of NaOH to reach the endpoint. What is the molarity of the base? Benzoic acid is monoprotic.