Mass percent concentration is defined as the mass of solute divided by the mass of solution times 100. Which option expresses this formula correctly?

• A. Mass percent = mass of solute divided by mass of solvent times 100
• B. Mass percent = mass of solute divided by mass of solution times 100
• C. Mass percent = mass of solvent divided by mass of solution times 100
• D. Mass percent = volume of solute divided by volume of solution times 100

Answer: (B)

Mass percent is a measure of how much of the total mass of a solution is the solute. The important point is that the denominator must be the mass of the entire solution (the sum of solute and solvent), not just the solvent itself. Therefore, the correct expression uses the mass of solute in the numerator and the total mass of the solution in the denominator, all multiplied by 100.

So it is mass of solute divided by mass of solution times 100. For example, if you have 2 g of solute and 8 g of solvent, the total solution mass is 10 g, and the mass percent is (2/10) × 100 = 20%.

The other forms aren’t correct for mass percent because using the solvent mass in the denominator would describe concentration by solvent, and using volumes instead of masses would not reflect mass percent at all.