In chemistry, most reactions are carried out in solutions. In order to perform calculation, it is necessary to know concentrations of the solutions. The quantity of solute dissolved in a given mass or volume of the solution or the solvent is called concentration of the solution. There are many ways to describe the concentration of a solution. The simplest way to express the concentration of a solution is in terms of mass or volume percentage of salute in solution.

**Mass percent (or Percent by mass):**

The mass of the solute person 100 mass units usually the gram unit of the solution is called mass percent of solute in the solution. For example, a 5% solution of sodium chloride means that 5 grams of sodium chloride are present in 100 grams of the solution.

Mathematically, mass percent of a solute in solution can be expressed as,

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Mass percent of the solute = ^{Mass of solute}**/** Mass of solution **× 100**

Both, the mass of solute and that of the solution is present in trace quantity (very small quantity), the concentration is expressed in the units of parts person million (abbreviation as ppm). For example, a sample containing of 5 grams of solute in 10^{6} grams of a solution is said to have a concentration of 5 ppm. The composition of gaseous mixtures is also expressed in ppm units based on the volume ratio. For example, 10 mL in 10^{6 }ml corresponds to 10 ppm.

**Volume percent (or Percent by volume):**

The number of units of volume of the solute person 100 units of volume of the solution is known as the volume percent (% v/v) in water. For example, a 5 %(v/v) solution of ethyl alcohol in water contains 5 ml of alcohol in 100 ml of the solution. Mathematically, volume percent is expressed as,

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Volume percent of the solute = ^{Volume of the solute} / _{Volume of the solution} **×** 100