When NaOH(s) (molar mass 40.0 g/mol) is heated, it decomposes according to the reaction above. If 80.0 g of a mixture containing both NaOH(s) and Na₂O(s) was heated and produced 0.400 mol of H₂O(g), what was the percent of NaOH(s) by mass within the mixture? (Assume that all the NaOH within the sample decomposed.)
