(a) Name the impurities present in hydrogen prepared in the laboratory.

(b) How can these impurities be removed?

(a) Impurities present are:

1. Hydrogen sulphide (H₂S)
2. Sulphur dioxide (SO₂)
3. Oxides of nitrogen
4. Phosphine (PH₃)
5. Arsine (AsH₃)
6. Carbon dioxide and
7. Water vapour

(b) The impurities can be removed from hydrogen by passing it through:

1. Silver nitrate solution [to remove arsine and phosphine].
   AsH₃ + 6AgNO₃ ⟶ Ag₃As + 3AgNO₃ + 3HNO₃
   PH₃ + 6AgNO₃ ⟶ Ag₃P + 3AgNO₃ + 3HNO₃
2. Lead nitrate solution [to remove hydrogen sulphide].
   Pb(NO₃)₂ + H₂S ⟶ PbS + 2HNO₃
3. Caustic potash solution [to remove sulphur dioxide, carbon dioxide and oxides of nitrogen].
   SO₂ + 2KOH ⟶ K₂SO₃ + H₂O
   CO₂ + 2KOH ⟶ K₂CO₃ + H₂O
   2NO₂ + 2KOH ⟶ KNO₂ + KNO₃ + H₂O
4. A drying agent used to dry the gas. Common drying agents such as fused calcium chloride, caustic potash stick and phosphorus pentoxide remove water vapour.

Thus, the gas is purified and dried and then collected over mercury because mercury has no reaction with it.