Upon completion of this topic, learners will be able to:

• express the concentration of a solution using different units.
• convert between the different concentration units.
• describe how to prepare solutions of specific concentration using pure solutes and solvent, or using solutions of higher concentration than the one desired.

Chemists use various ways to express concentration quantitatively. Let us examine the most common units of concentration: %w/w, %v/v, ppm, ppb, mole fraction, molarity, normality, and molality.

Percentage

Percentage unit of concentration refers to the percentage of solute present in a solution. The percentage of solute can be expressed by mass or by volume.

Mass percentage :- mass/mass (%w/w)

It is the number of grams of solute in 100 grams of solution. Calculation of this ratio is carried out by using the following formula:

Volume Percentage:

The volume percent is the volume of a liquid in 100 mL of solution. The label on a bottle of ordinary rubbing alcohol reads “isopropyl alcohol, 70% by volume.”

Such a solution can be made by mixing 70 mL of alcohol with water to make a total volume of 100 mL.

Molarity

Molarity is the most common concentration measure for aqueous solutions. It is a concentration unit defined as 'number of moles of solute dissolved in one liter of solution.'

One mole of NaOH has a mass of 40.0 g. If this quantity of NaOH is dissolved in enough water to make exactly 1.00 L of solution, the solution is a 1 M solution. This relationship between molarity, moles, and volume is expressed in the following way.

Molality

For applications where the solution temperature changes, chemists prefer to use molality (m, units of mol/kg).

Molality is the concentration of a solution expressed in moles of solute per kilogram of solvent. A solution that contains 1 mol of solute, (sodium hydroxide, NaOH, for example), dissolved in exactly 1 kg of solvent is a “one molal” solution.

The molality of any solution can be found by dividing the number of moles of solute by the mass in kilograms of the solvent in which it is dissolved.