Which of the following statements about concentrations of solutions is true?

• A. Molality is defined as moles of solute per liter of solution.
• B. Molarity is defined as moles of solute per liter of solution.
• C. Mole fraction is always greater than 1 for any solute in a solution.
• D. Percent by mass is calculated as grams of solute divided by liters of solution, multiplied by 100.

Answer: (B)

Verified step by step guidance

1. Step 1: Understand the definitions of concentration units. Molarity (M) is defined as the number of moles of solute divided by the volume of the solution in liters. The formula is \(M = \frac{\text{moles of solute}}{\text{liters of solution}}\).
2. Step 2: Review the definition of molality (m). Molality is the number of moles of solute per kilogram of solvent, not per liter of solution. The formula is \(m = \frac{\text{moles of solute}}{\text{kilograms of solvent}}\).
3. Step 3: Consider the mole fraction. Mole fraction is the ratio of moles of one component to the total moles in the solution. Since it is a fraction of the total, it must always be less than or equal to 1, never greater than 1.
4. Step 4: Analyze percent by mass. Percent by mass is calculated as the mass of solute divided by the total mass of the solution, multiplied by 100. It is not based on volume, so grams of solute divided by liters of solution is incorrect.
5. Step 5: Conclude that the only true statement is that molarity is defined as moles of solute per liter of solution.