Which expression is a valid way to express concentration using mass percent?

• A. Molarity
• B. Mass percent
• C. Mole fraction
• D. Normality

Answer: (B)

Concentration expressed as a mass percent uses a mass-to-mass ratio. It is defined as the mass of solute divided by the total mass of the solution, multiplied by 100 percent. This makes it a straightforward way to describe how much solute is present by mass in the mixture.

For example, if there are 5 g of solute in a total solution mass of 100 g, the mass percent is (5 g / 100 g) × 100% = 5%.

This differs from other concentration expressions. Molarity uses moles per liter and depends on volume, mole fraction uses moles per total moles, and normality uses equivalents per liter. Mass percent stays within mass ratios, which is why it’s the correct expression when phrased in terms of mass percent.