Solute, Solvent and Solution

If a substance dissolves in a liquid, it is called a solute and the liquid is called the solvent. The resulting mixture is called a solution.

Solute + Solvent = Solution


Solute Solid which dissolves
Solvent Liquid which does the dissolving
Solution Solute + solvent
Saturated solution A solution which will dissolve no more solute.
Soluble substance A substance that will dissolve in a solvent.
Insoluble substance A substance that will not dissolve in a solvent

Concentration of a Solution

  1. The concentration of a solution tells you how much solute is dissolved in 1 unit volume of solution.
  2. The volume of a solution is measured in dm³ (litres).  1 dm³ = 1000 cm³.
  3. The amount of solute can be measured in grams or moles.
  4. 2 units of concentration used in chemistry are g dm-3 and mol dm-3

Concentration in g dm-3

  1. Concentration is the number of moles of solute per liter of solution.
  2. A concentration of 10 g dm-3 means there is 10 g of solute dissolved in1 dm3 of solution.

Concentration = Mass of solute(g) Volume of solution(dm 3 )

Example 1:

Calculate the concentration of the solution if 28g of NaOH is dissolve in 250cm3 of water.

Answer:

Mass of solute = 28g
Volume of solvent = 250cm³ = 0.25dm³

Concentration= Mass/Volume = 28g/0.25dm3 = 112g/dm3

Concentration in mol dm-3 (Molarity)
  1. Molarity is probably the most commonly used unit of concentration. It is the number of moles of solute per liter of solution.
  2. A concentration of 2 mol dm-3 means there are 2 moles of solute dissolved in 1 dm3 of solution.

Molarity = Mole of solute(mol) Volume of solution(dm 3 )

Example:

What is the molarity of a solution made when water is added to 0.2 mol of CaCl2 to make 100 cm³ of solution? [RAM: Ca = 40; Cl = 35.5]

Answer:

Number of mole of solute = 0.2 mol
Volume of solvent = 100 cm³ = 0.1 dm³

Molarity = Number of Mole/Volume = 0.2mol/0.1dm3 = 2mol/dm3

Conversion of the Unit of Concentration

  1. The chart above shows how to convert the units of concentration from g dm-3 to mol dm-3 and vice versa.
  2. The molar mass of the solute is equal to the relative molecular mass of the solute.

Example 1:
The concentration of a Potassium chloride solution is 14.9 g dm-3. What is the molarity ( mol dm-3) of the solution? [ Relative Atomic Mass: Cl = 35.5; K = 39 ]

Answer:
Relative Formula Mass of Potassium Chloride (KCl)
= 39 + 35.5 = 74.5

Molar Mass of Potassium Chloride = 74.5 g/mol

Molarity of Potassium Chloride
Molarity = Concentration Molar Mass = 14.9gd m −3 74.5gmol−1 =0.2mol/dm3

Example 2
A solution of barium hydrokxide have molarity 0.1 mol dm-3. What is the concentration of the solution in g dm-3? [Relative Atomic Mass: Ba = 137; O = 16; H = 1 ]

Answer:
Relative Formula Mass of barium hydrokxide, Ba(OH)2
= 137 + 2(16+1) = 171

Molar Mass of Potassium Chloride = 171 g/mol
Concentration

= Molarity×Molar Mass

=0.1mol/dm3 × 171gmol−1 =17.1gmol−1