0.4M is the concentration of a solution formed when 4.00 grams of sodium hydroxide is dissolved to make a 250cm cubed solution.
Molar concentration, also known as molarity, quantity concentration, or substance concentration. Molarity is a unit used to describe the amount of a substance in a solution expressed as a percentage of its volume. The number of moles per liter, denoted by the unit symbol mol/L or mol/dm³ in SI units, is the most often used unit denoting molarity. Because the volume of most solutions very minimally changes with temperature owing to thermal expansion, using molar concentration in thermodynamics is frequently not practical.
The formula for molarity,
Molarity = (moles/vol) x 1000
= x 1000
= (0.1 / 250) x 1000
=0.4M
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