Define molarity.

Molarity specifically refers to a way of expressing concentration in terms of the amount of substance. It is defined as the number of moles of solute present in one liter of solution. This is important because molarity provides a clear and standardized method to quantify the concentration of solutes, which is crucial for stoichiometric calculations in chemistry.

When discussing reactions, it is often necessary to know how much of a solute is present in a given volume of solution to predict the outcome of chemical reactions or to prepare solutions of desired concentrations. Molarity allows chemists to easily convert between moles and volume, facilitating these calculations in laboratory and industrial settings.

In contrast, the other options describe different concepts or measurement approaches. For instance, mass of solute per volume of solvent does not account for the total volume of the solution, and total concentration of all solutes in solution could refer to a separate measure called molality or molar concentration. The number of molecules per milliliter also does not utilize the standard molar measurement method, as it does not convert molecules to moles, which is fundamental in chemistry.