The number of units, which are used to express concentration of solutions from the following is _____.
(Mass percent, Mole, Mole fraction, Molarity, ppm, Molality.)

The concentration of a solution is the amount of solute present in a given amount of solvent or solution. Therefore, only those quantities that express a ratio of solute to solvent/solution can be used as concentration units.

Mass percent $$(w/w\%)$$ is defined as the mass of solute per 100 parts by mass of solution and is a valid concentration term.

Mole is the SI unit of amount of substance. It represents only the quantity of particles and does not express any ratio. Hence, it is not a concentration term.

Mole fraction $$(x)$$ is the ratio of the moles of a component to the total moles present in the solution and is a valid concentration term.

Molarity $$(M)$$ is the number of moles of solute present in one litre of solution and is a concentration term.

ppm (parts per million) expresses the number of parts of solute per million parts of solution and is commonly used for very dilute solutions. It is also a concentration term.

Molality $$(m)$$ is the number of moles of solute present per kilogram of solvent and is a concentration term.

Thus, the valid concentration terms are:

• Mass percent

• Mole fraction

• Molarity

• ppm

• Molality

"Mole" is the only quantity that does not represent concentration.

Hence, the number of terms used to express concentration is

$$\boxed{5}$$