Which of the following is NOT used for expressing the concentration of a solution? 

Correct Answer: Volume by Mass percentage of a solution

Explanation:

Concentration of a solution is defined as the amount of solute present in a given quantity of solution or solvent. In chemistry, standard methods for expressing concentration based on mass and volume ratios include:

1. Mass by Mass percentage (w/w %): It represents the mass of solute in grams present in 100 grams of the solution.

$\text{Mass \%}=\frac{\text{Mass of solute}}{\text{Mass of solution}}\times 100$

2. Volume by Volume percentage (v/v %): It represents the volume of solute in mL present in 100 mL of the solution.

$\text{Volume \%}=\frac{\text{Volume of solute}}{\text{Volume of solution}}\times 100$

3. Mass by Volume percentage (w/v %): It represents the mass of solute in grams present in 100 mL of the solution.

$\text{Mass by Volume \%}=\frac{\text{Mass of solute}}{\text{Volume of solution}}\times 100$

Volume by Mass percentage (expressing volume of solute per unit mass of solution) is not a standard or accepted unit for expressing the concentration of a solution in general chemical analysis.