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.