What is the number of moles of the solute in 250 ml of a.4 m solution?

There are 0.1 moles of solute in 250 mL of 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.

250mL⋅1L1000mL⋅0.4mol1L

Units of liters and milliliters cancel, leaving you with a final answer in units of moles, at 0.1 mol.