Question

A student wants to prepare exactly 250 ml of a 0.500 m aqueous potassium hydroxide solution. what mass of potassium hydroxide (molar mass = 56.10 g/mol) must the student dissolve in the 250 ml of solution?

Answer 1

Molarity is defined as the number of moles of solute in volume of 1 L solution.
the molarity of KOH solution to be prepared is 0.500 M
the volume of solution to be prepared is 250 mL
the number of KOH moles in 1 L - 0.500 mol 
therefore number of KOH moles in 250 mL - 0.500 mol/L x 0.250 L = 0.125 mol
Mass of KOH in 0.125 mol - 0.125 mol x 56.10 g/mol = 7.01 g
The mass of KOH required to make the solution is therefore 7.01 g