a) Define the following in term:. of electron transfer: (i) oxidation; (ii) reduction.
(b)(i) Determina the oxidation stale of phosphorus in each of the following structures: I. POCI\(_3\) II. PH\(_3\).
(ii) State with reasons whether the following compounds will form acidic, neutral or basic aqueous solutions: I. NaNO\(_3\) II. Na\(_2\)H\(_4\)CI; Ill. Na\(_2\)CO\(_3\).
(v) Give of collection of the gas one I. physical property; II. chemical property of the gas
(vi) State one chemical test to identify the gas.
(d) A 4.3 g hydrated sodium tetraoxosulphate (VI) (Na\(_2\)SO\(_4\).xH\(_2\)O) was heated to remove the water of crystallization. The remaining anhydrous salt had a mass of 2.12 g. Calculate the value of x in the t I hydrated salt. [H = 1; O = 16; Na = 23; S = 32 ]
(a) Definitions in terms of electron transfer
(i) Oxidation is the loss of electrons by an atom, ion or molecule.
(ii) Reduction is the gain of electrons by an atom, ion or molecule.
(b)(i) Oxidation state of phosphorus
I. POCl3: taking O = \(-2\) and Cl = \(-1\), \[x + (-2) + 3(-1) = 0 \Rightarrow x = +5.\] Phosphorus is \(+5\).
II. PH3: phosphorus is more electronegative than hydrogen, so each H is \(+1\), \[x + 3(+1) = 0 \Rightarrow x = -3.\] Phosphorus is \(-3\).
(b)(ii) Nature of the aqueous solutions (with reasons)
- NaNO3 is neutral. It is the salt of a strong base (NaOH) and a strong acid (HNO3), so neither ion hydrolyses.
- NH4Cl is acidic. It is the salt of a strong acid (HCl) and a weak base (NH3); the ammonium ion hydrolyses to release H+.
- Na2CO3 is basic. It is the salt of a strong base (NaOH) and a weak acid (H2CO3); the carbonate ion hydrolyses to release OH-.
(c) The set-up (a flask containing NaCl to which concentrated H2SO4 is added and heated; the gas is passed through concentrated H2SO4 in the conical flask and then collected in an upright gas jar).
(i) Gas produced: hydrogen chloride gas, HCl. \[\text{NaCl} + \text{H}_2\text{SO}_4 \rightarrow \text{NaHSO}_4 + \text{HCl}\uparrow\]
(ii) Method of collection: upward displacement of air (downward delivery, into an upright jar).
(iii) Reason: hydrogen chloride is denser than air and is very soluble in water, so it cannot be collected over water and is collected by displacing air upward.
(iv) Function of the concentrated H2SO4 in the conical flask: it dries the gas by absorbing the water vapour (moisture) carried along with the hydrogen chloride.
(v)
I. Physical property: it is a colourless gas with a sharp, choking (pungent) smell and it fumes in moist air.
II. Chemical property: it is acidic; it turns damp blue litmus paper red and reacts with ammonia to form dense white fumes of ammonium chloride.
(vi) Chemical test: hold a glass rod moistened with concentrated ammonia solution near the gas; dense white fumes of NH4Cl confirm hydrogen chloride. \[\text{HCl} + \text{NH}_3 \rightarrow \text{NH}_4\text{Cl}\]
(d) Value of x in Na2SO4.xH2O
Mass of water driven off: \[4.3 - 2.12 = 2.18\ \text{g}\]
Molar masses: \(\text{Na}_2\text{SO}_4 = 2(23)+32+4(16) = 142\); \(\text{H}_2\text{O} = 18\).
\[n(\text{Na}_2\text{SO}_4) = \frac{2.12}{142} = 0.0149\ \text{mol}\]
\[n(\text{H}_2\text{O}) = \frac{2.18}{18} = 0.1211\ \text{mol}\]
\[x = \frac{n(\text{H}_2\text{O})}{n(\text{Na}_2\text{SO}_4)} = \frac{0.1211}{0.0149} \approx 8\]
Therefore \(x \approx 8\); the hydrated salt is approximately Na2SO4.8H2O.
