Percent by mass is defined as what?

• A. % m/m = (mass solute / mass solution) × 100
• B. % m/m = (mass solvent / mass solution) × 100
• C. % m/m = (mass solute / mass solvent) × 100
• D. % m/m = (mass solute / mass solution) × 10

Answer: (A)

Percent by mass measures what fraction of the total mass is the dissolved substance. It is defined as the mass of solute divided by the mass of the entire solution, multiplied by 100. Since the solution’s mass is the sum of solute and solvent, you can compute it from those quantities: percent by mass = (mass solute / mass solution) × 100.

For example, dissolving 2 g of solute in 8 g of solvent gives a total solution mass of 10 g, so the percent by mass is (2 g / 10 g) × 100 = 20%.

The other forms don’t fit the definition: using mass of solvent in the numerator gives the percent by mass of the solvent, not the solute; using mass solute over mass solvent is a ratio to the solvent, not to the total solution; and multiplying by 10 instead of 100 yields the wrong scale.