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Answer: The correct answer is A. Molality. It is the concentration expressed as moles of solute per kilogram solvent. It is most commonly used in solving problems involving Colligative Properties (e.g. boiling point elevation, freezing point depression). Other ways to express concentration are Molarity, Mole Fraction, Parts per Million, Parts per Billion, and Normality.

Further Explanation

B. Mole Fraction is the ratio of the moles of a solute to the total moles of the solution. This unit of concentration is expressed as:

[tex]mole\ fraction \ of \ x = \frac{moles \ of \ solute \ x}{total\ moles \ of \ solution}[/tex]

C. Molarity is the most common ways of expressing concentration. It is the amount of solute per liter of solution and is expressed as:

[tex]Molarity\ (M) \ = \frac{moles \ of \ solute}{liter \ of \ solutiont}[/tex]

D. ppm (or parts per million) is used for very dilute solutions. It is expressed as the mass of solute in milligrams per liter of the solution:

[tex]parts\ per \ million \ (ppm) = \frac{mass\ of \ solute \ (in\ mg)}{liter\ of\ solution}[/tex]