7.1 A learner dissolves ammonium chloride (NH₄Cl) crystals in water and measures the pH of the solution - NSC Physical Sciences - Question 7 - 2016 - Paper 2

The pH of the solution will be SMALLER THAN 7. This is because ammonium ions (NH₄⁺) produced during hydrolysis react with water to form hydronium ions (H₃O⁺). The chemical equation for this reaction is:

$ext{NH}_4^+(aq) + ext{H}_2O(l) \rightarrow ext{NH}_3(aq) + ext{H}_3O^+(aq)$

The production of H₃O⁺ ions leads to an increase in acidity, thus lowering the pH below 7.

To calculate the number of moles of H₂SO₄, we use the formula:

$n = \frac{m}{M}$

Where:

• $m$ = mass of H₂SO₄ = 7.35 g
• $M$ = molar mass of H₂SO₄ = 98 g/mol

Substituting the values:

$n = \frac{7.35g}{98g/mol} = 0.075mol$

Therefore, the number of moles of H₂SO₄ present in the solution is 0.075 moles.

From the balanced equation:

$ext{H}_2 ext{SO}_4(aq) + 2 ext{NaOH}(s) \rightarrow ext{Na}_2 ext{SO}_4(aq) + 2 ext{H}_2 ext{O}(l)$

It can be observed that 1 mole of H₂SO₄ reacts with 2 moles of NaOH. Thus, the number of moles of NaOH required is:

$n( ext{NaOH}) = 2 imes n( ext{H}_2 ext{SO}_4) = 2 imes 0.075mol = 0.15mol$

To find the mass of NaOH, we use:

$m = n \times M$

Where:

• $M$ of NaOH = 40 g/mol

Thus,

$m( ext{NaOH}) = 0.15mol \times 40g/mol = 6g$

Therefore, the mass of NaOH added to the H₂SO₄ solution is 6 g.