Question

A 25.0-g sample of sodium hydroxide is dissolved in 400.mL of water. What is the concentration of the solution?

Answer 1

In what? % w/w; % w/v, mole fraction, M, m......? If you want it in m, then
mols = grams/molar mass = ?
Then m = mols NaOH/kg solvent.
You can't do % w/v or M.

Answer 2

[NaOH] = [(25g)/(40g/mole)]/(0.400Liters) = 1.56M

Answer 3

The primary reason I said M was not possible is because that definition is mols/L SOLUTION. Your problem gives 400 mL water which is not the same as 400 mL solution. 25 g NaOH + 400 mL water is not 400 mL solution but more than that. By what volume who knows. So you may calculate m but not M.