Calculate mass of solid needed for preparation of 500 mL 1 M solution of oxalic acid.

Definition 6.1 of molar concentration can be rearranged to form:

mpq1.2

So to prepare the solution we will need 0.5×1=0.5 mole of oxalic acid (0.5 L is 500 mL).

Molar mass of oxalic acid is 90.03 g, so we have to use 0.5×90.03=45.02 g of solid.

This answer, although seems correct, ignores one important fact. Oxalic acid crystallizes in the form of dihydrate with molar mass 126.07, so to prepare solution we have to weight 63.03 g of solid. To avoid such mistakes you have to either read labels on the bottles with reagents (they should contain information about reagent composition, often also about molar mass), have really good memory, or use solution preparation program with built in database. Note that it is the same program we used checking results or solving other concentration questions.