Problem #27 Concentration [Solved]

1. Explain or define the concept of concentration.

2. Explain different ways, if any, of expressing the concentration.

Explanations

1. Concentration: is the amount or quantity of a dissolved substance, solute, per unit quantity of solution or mixture. The symbol of concentration is C.

2.There are many ways of expressing the concentration, the most common are:

2.1. Molarity: the concentration is expressed as the Number of Moles of the Solute per Littre of Solution. The units of Molarconcentration are: mol/dm³, mol/L, M.

Example: If 40g (40g/40g.mol^- = 1 mol) of NaOH are dissolved in distilled water to make 1 L of solution, the concentration of that solution is: 1 mol/dm³= 1 mol/L = 1M

This form of expressing the concentration is very popular in Chemistry.

2.2. Molality: is the Amount of Solute per Unit Mass of Solvent, commonly given in units of mol/Kg of solvent. The units of molal concentration are: mol/Kg of solvent or m (please notice the small difference with the definition of Molarity).

Example: If 40 g of NaOH is dissolved in 1Kg of distilled water, the molal concentration of that solution is: 1 mol/Kg of solvent = 1m

2.3. Normality: a solution 1 normal has a concentration of one gram equivalent per dm³ or L of solution.

Gram equivalent or Equivalent weight is the mass of an element or compound that could combine with or displace one gram of hydrogen in a chemical reaction. The equivalent weight represents the combining power of a substance.

The normality of a solution is represented by the symbol N

N= normality, Eq = number of gram equivalent of the solute= molar (formula unit) mass/gram equivalent, V= volume of the solution in L.

Examples of gram equivalents:

If you know the Molarity (M) of an acid or base solution, you can easily convert it into Normality (N) by multiplying by the number of hydrogen (or hydroxide) ions in the acid (or base) liberated in the reaction:

Examples: 2M H₂SO₄ = 4N H₂SO₄

1M NaOH = 1N NaOH

2.4. Percent (%) concentration:

There are 3 ways of expressing the percent concentration.

(i) Percent mass/mass, %m/m or %w/w: the mass or weight of a solute or component in g/ 100 g of the sample, multiply by 100.

Example

(ii) Percent volume/volume, %v/v: the volume of the solute in mL/100 mL of the solution, multiply by 100.

Example: A bottle of beer marked 5%v/v alcohol: means that in each 100 mL of the beer there is 5 mL of alcohol: (5 mL/100 mL) x 100 = 5%.

(iii) Percent mass /volume, %m/v or %w/v: the mass of the solute in 100 mL of the solution, multiply by 100.

Example: A saturated aqueous solution of NaCl contains 23 g of NaCl in 100 mL of solution; therefore its concentration is: 23%w/v.

Percent (%) concentration is commonly found on the labels of many consumable products and medicines (pharmaceutical products).

2.5. Parts per million (ppm): indicates the mass of a chemical or contaminate per unit volume of water or air; it is expressed in mg/L. This unit is mainly used in environmental studies and measurements, where minute amounts of contaminates are dealt with: 1ppm = 1 mg/Kg = 1mg/L.

Example: In USA, according to the Safe Drinking Water Act, EPA set the level of Lead in drinking water at 0.015 ppm (0.015 mg/L)

2.6. Mole fraction: the mole fraction of a solute in a solution is the number of moles of the solute divided by the total number of moles in the solution; it has no units.

Mole fraction = = (Number of moles of the solute/Total number of moles in the solution)

Example: In a gaseous mixture of 5 moles of N₂ and 2 moles of O₂, the nitrogen mole fraction is 5/7, whereas the oxygen mole fraction is 2/7.