Stoichiometric Calculations Involving Reactions/Solutions

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Most chemical reactions are carried out in solutions. It is therefore important to know how to express the concentration of the substance in its solution. Most common ways of expressing concentration of any solute in a solution are:

Mass Percent

• is the mass of the solute in grams per 100 grams of the solution.

A 5 % solution of sodium chloride means that 5 g of NaCl is present in 100g of the solution.

Volume percent

• is the number of units of volume of the solute per 100 units of the volume of solution.

A 5 % (v/v) solution of ethyl alcohol contains 5 cm³ of alcohol in 100 cm³ of the solution.

Molarity and Molarity Equation

Molarity

• of the solution is defined as the number of moles of solute dissolved per litre (dm³) of the solution. It is denoted by the symbol M. Measurements in Molarity can change with the change in temperature because solutions expand or contract accordingly.

The Molarity of the solution can also be expressed in terms of mass and molar mass

In terms of weight, molarity of the substance can be expressed as:

The units of molarity are moles per liter (mol L^-1).

Molarity equation

To calculate the volume of a definite solution required to prepare solution of other molarity, the following equation is used:

M₁V₁ = M₂V₂, where M₁= initial molarity, M₂= molarity of the new solution, V₁= initial volume and V₂= volume of the new solution.

The molarity equation is commonly used to calculate the molarity of solutions after dilution.