Molarity and Number of Moles

Number of mole of solute in a solution can be calculated by using the following formula
\[n = \frac{{MV}}{{1000}}\]
n = number of mole of solute
M = molaruty of the solution
V = volume of the solution in cm3

Example:
How many moles of zinc sulphate is present in 200cm3 of 0.1 mol dm-3 zinc sulphate solution?

Answer:
Molarity, M = 0.1 mol dm-3
Voloume, V = 200cm3
\[\begin{gathered}
n = \frac{{MV}}{{1000}} \hfill \\
n = \frac{{(0.1)(200)}}{{1000}} \hfill \\
n = 0.02mol \hfill \\
\end{gathered} \]

Molarity and the pH Value of Acid or Alkali

  1. The table below shows the pH value and the molarity of a strong acid and a weak acid.
    Molarity of Acids
    pH value
    Hydrochloric acid
    Ethanoic Acid
    0.1 mol dm-3
    1.0
    2.9
    0.01 mol dm-3
    2.0
    3.4
    0.001 mol dm-3
    3.0
    3.9
    0.0001 mol dm-3
    4.0
    4.4
    0.00001 mol dm-3
    5.0
    4.9
  2. The table below shows the pH value and the molarity of a strong alkali and a weak alkali.
    Molarity of alkali
    pH value
    Sodium hydroxide solution
    Ammonia aqueous
    0.1 mol dm-3
    13.0
    11.1
    0.01 mol dm-3
    12.0
    10.6
    0.001 mol dm-3
    11.0
    10.1
    0.0001 mol dm-3
    10.0
    9.6
    0.00001 mol dm-3
    9.0
    9.1
  3. According to the tables shown above, we can conclude that the pH value of acid or alkali is affected by 2 factors:
    1. The Molarity
    2. Strong or weak acid(Alkali)

Concentration of Acid

  1. We have learned that the acidity of a solution is owing to the presence of hydrogen ions in a solution.
  2. We need to distinguish between the concentration of acid and the concentration of hydrogen ions of an acid.
  3. In most cases, the concentration of hydrogen ions has greater significant than the concentration of the acid.
  4. The concentration of hydrogen ion in acid depends on the
    1. molarity (concentration) of the acid
      The higher the molarity of an acid, the higher the concentration (or molarity) of the hydrogen ions in the acid
    2. strength of the acid
      Strong acid has higher concentration of hydrogen ions compare with weak acid of same concentration and same basicity
    3. basicity of the acid
      The concentration of hydrogen ions of a diprotic acid is higher than the monoprotic acid and lower than the triprotic acid

Example:

Calculate the number of mol of hydrogen ions that contain in 200cm3 of sulphuric acid 0.4 mol dm-3.

Answer:

Number of mole of sulphuric acid
\[\begin{gathered}
n = \frac{{MV}}{{1000}} \hfill \\
n = \frac{{(0.4)(200)}}{{1000}} \hfill \\
n = 0.08mol \hfill \\
\end{gathered} \]

Sulphuric acid is a diprotic acid. Each molecule of sulphuric acid will produce 2 hydrogen ions when dissolve in water.

Number of mole of hydrogen ions
= 2 x Number of mole of sulphuric acid
= 2 x 0.08 = 0.16 mol