What is the mass of 2.50 * 1022 molecules of NaOH (Molar mass = 40.0 g/mol) ?

Two things are given to us: The "molar mass" of NaOH and the number of molecules of NaOH.

The molar mass is different from "mass"; 1 mole of NaOH is equal to 40.0g/mol.

When the question asks, what is the mass? It is referring to how many grams there are.

First, we are going to convert the molecules to atoms using Avogadro's number (=6.02 x 10^23)

(2.50 x 10^22 molecules) x (6.02 x 10^23 atoms) = 0.042 mol

Next, we are going to convert from moles to grams.

0.042 mol of NaOH x 40.0 g of NaOH/1 mole = 1.68g of NaOH.

There are 1.68 grams in 2.50 x 10^22 molecules of NaOH.