What is the number of moles of solute in 250 mL of a 0.4 M solution?
First, recognize that the molar concentration tells you how many moles of the solute are present in one liter of solution. In a 0.4 M solution, there are 0.4 moles of solute in every liter of solution. You can determine the number of moles of solute in 250 mL of the solution using dimensional analysis.
Units of liters and milliliters cancel, leaving you with a final answer in units of moles, at 0.1 mol.