What is the concentration of a solution if I dissolve 42g of NaOH in a total volume of 375ml of solution
Do you want % w/w. % w/v, M, N, just what. For molarity you do this.
mols NaOH = grams/molar mass.
Then M = mols/L soln.
mols NaOH = grams/molar mass.
Then M = mols/L soln.