(a)(i) Oxidation is the process of electron loss. Reduction is the process of electron gain
(b)(i) POCI${}_3$
O.N of P + O.N of oxygen + (O.N of CI) x 3 = 0
P + —2 + —1 x 3 = 0
—P — 2 — 3 = 0
P = — 5
(ii) I. NaNO${}_3$ is neutral because it is a salt of a strong acid and a strong base, its ions do not undergo hydrolysis.
II. NH${}_4$CI is acidic because it is a salt of a strong acid and a weak base. The NH${}_4^{+}$ undergoes hydrolysis.
III. Na${}_4$CO${}_3$ is basic because it is a salt of weak acid and ntrong base. The CO${}_3^{2-}$ undergoes hydrolysis.

(c)(i) Hydrogen chloride gas
(ii) downward delivery or upward displacement of air.
(iii) The gas is denser than air
(iv) drying the gas.
(v) Physical properties
— denser than air — very soluble in water — colourless gas — fumes in moist air
Chemical Properties
— gives cloud or white fumes in contact with ammonia vapour
(vi) Pass the gas into AgNO${}_{3(aq)}$ a white precipitate of AgCl is formed wihich is soluble in aqueous ammonia but insoluble in HNO${}_3$

(d) Mass of water = 4.30 — 2.12 = 2.18g
Mass of Na${}_2$SO${}_4$ = 23 x 2 + 32 + 16 x 4 = 142
Molar mass of H${}_2$O = 2 + 16 = 18
2.12g of anhydrous salt contain 2.18g of water
142g of anhydrous salt contain $\frac{2.18\times 142}{2.12}$
= 146.018
X(H${}_2$O) = 146
18x = 146
x = $\frac{146}{18}$
= 8