A solution contains 2.0 moles of solute in 1.0 liter of solution. What is its molarity?

• A. 1.0 M
• B. 2.0 M
• C. 0.5 M
• D. 0.25 M

Answer: (D)

Molarity is the amount of solute in moles per liter of solution. Here, you have 2.0 moles of solute in 1.0 liter of solution, so the concentration is 2.0 moles per liter, i.e., 2.0 M. This uses the final volume of the solution in the denominator, not the volume of solvent. If the volume were different, the molarity would change accordingly. The given numbers correspond to 2.0 M; the other values would require different combinations of moles and volume (for example, 1.0 M would be 1.0 mole per liter, 0.5 M would be 1.0 mole in 2.0 liters, etc.).