Question

A student has 470.0 mL of a 0.1429 M aqueous solution of Na2SO4 to use in an experiment. He accidentally leaves the container uncovered and comes back the next week to find only a solid residue. The mass of the residue is 21.64 g. Determine the chemical formula of this residue.

If the substance is a hydrate, use a period instead of a dot in its formula.