Applied and Analytical Chemistry

(c) Chlorofluorocarbons

Photochemical smog forms from nitrogen oxides and hydrocarbons in sunlight, giving ozone, PAN (peroxyacetyl nitrate) and acrolein. Chlorofluorocarbons are not part of smog; they are inert at ground level and damage the stratospheric ozone layer.

(d) 37.33

Moles $\mathrm{H_2SO_4}=1\times0.010=0.01$ mol, which neutralises $2\times0.01=0.02$ mol $\mathrm{NH_3}$. Mass of N $=0.02\times14=0.28$ g. $\%\,\mathrm{N}=\dfrac{0.28}{0.75}\times100=37.33$.

(d) $\mathrm{Na_2S>ZnS>CuS}$

$\mathrm{Na_2S}$ is an ionic alkali metal salt and is freely soluble in water. ZnS is sparingly soluble ($K_{sp}\sim10^{-25}$) and CuS is far less soluble ($K_{sp}\sim10^{-36}$), so the order is $\mathrm{Na_2S>ZnS>CuS}$.

(b) Micro-organisms present in the soil

Soil micro-organisms oxidise CO to $\mathrm{CO_2}$ and so act as the main natural sink for carbon monoxide. Haemoglobin binds CO (forming carboxyhaemoglobin), but this is poisoning, not removal from the atmosphere. Oceans and plants are not significant sinks for CO.

(a) $\mathrm{N_2O_5}$

NO, $\mathrm{NO_2}$ and $\mathrm{N_2O}$ are all released into the atmosphere by both natural processes (lightning, soil bacteria) and human activity (combustion, fertilisers). $\mathrm{N_2O_5}$ is not a common primary pollutant from these sources; it forms only as an intermediate in atmospheric nitrogen oxide chemistry.

(c) Sulphur dioxide

Methane, ozone and nitrous oxide absorb infrared radiation re-emitted by the Earth and trap heat, so they are greenhouse gases. $\mathrm{SO_2}$ is an acid-rain pollutant and is not counted as a greenhouse gas.

(a) Clark's method

Temporary hardness is due to $\mathrm{Ca(HCO_3)_2}$ and $\mathrm{Mg(HCO_3)_2}$. In Clark's method, adding slaked lime precipitates them as $\mathrm{CaCO_3}$ and $\mathrm{Mg(OH)_2}$. Ion-exchange, synthetic resin and Calgon methods remove both temporary and permanent hardness. Clark's method is the one specific to temporary hardness.

(c) (a)-(iv), (b)-(iii), (c)-(ii), (d)-(i)

Pure nitrogen is obtained by heating sodium or barium azide. The Haber process makes ammonia, the Contact process makes sulphuric acid, and the Deacon process makes chlorine by oxidising HCl with air over $\mathrm{CuCl_2}$.

(a) The carboxyhaemoglobin (haemoglobin bound to CO) is less stable than oxyhaemoglobin.

This statement is the incorrect one. CO binds haemoglobin about 200-300 times more strongly than $\mathrm{O_2}$, so carboxyhaemoglobin is far more stable than oxyhaemoglobin. Because CO displaces $\mathrm{O_2}$, the oxygen-carrying capacity of blood falls (option d). CO forms by incomplete combustion (option b).

(a) A-IV, B-III, C-I, D-II

$\mathrm{SO_2}$ oxidation to $\mathrm{SO_3}$ occurs in the troposphere (tropospheric pollution). Photolysis of HOCl releases Cl, which depletes ozone. Carbonate reacting with $\mathrm{H_2SO_4}$ is damage from acid rain. Photolysis of $\mathrm{NO_2}$ to NO and O starts photochemical smog formation.

(b) (i), (ii), (iv) only

Particulate matter catalyses the oxidation of $\mathrm{SO_2}$ to $\mathrm{SO_3}$, and so do the oxidants ozone and hydrogen peroxide, which convert $\mathrm{SO_2}$ to sulphuric acid. Hydrocarbons are not involved in this enhancement.

(d) $\mathrm{[Fe(SCN)]^{2+}}$

When both N and S are present, fusion with sodium gives NaSCN. With $\mathrm{Fe^{3+}}$ it forms the blood red $\mathrm{[Fe(SCN)]^{2+}}$. Prussian blue $\mathrm{Fe_4[Fe(CN)_6]_3}$ (option a) is the test for nitrogen alone, and $\mathrm{[Fe(CN)_5NOS]^{4-}}$ is the violet complex of sulphide with nitroprusside.

(d) Statement I is incorrect but Statement II is true

Eutrophication is caused by nutrient-rich (excess N and P) water bodies, not nutrient-deficient ones, so Statement I is false. The resulting algal bloom decomposes and depletes dissolved oxygen, so Statement II is true.

(d) B, A, D, C, E

In qualitative analysis, $\mathrm{Cu^{2+}}$ is in Group II (precipitated as CuS with $\mathrm{H_2S}$ in acid). $\mathrm{Al^{3+}}$ is in Group III ($\mathrm{NH_4OH/NH_4Cl}$). $\mathrm{Co^{2+}}$ is in Group IV ($\mathrm{H_2S}$ in basic medium). $\mathrm{Ba^{2+}}$ is in Group V (carbonates) and $\mathrm{Mg^{2+}}$ is in Group VI. The order is therefore Cu, Al, Co, Ba, Mg.

(c) Dilute sulphuric acid

Mohr's salt is $\mathrm{FeSO_4\cdot(NH_4)_2SO_4\cdot6H_2O}$. A little dilute $\mathrm{H_2SO_4}$ suppresses hydrolysis of $\mathrm{Fe^{2+}}$ and also supplies the common sulphate ion. Nitric acid is wrong because it oxidises $\mathrm{Fe^{2+}}$ to $\mathrm{Fe^{3+}}$. Concentrated acid is also unsuitable, and HCl would introduce a different anion.

(b) $\mathrm{2CuO + C \xrightarrow{\Delta} 2Cu + CO_2}$

Lassaigne's test fuses the compound with sodium to give NaX, NaCN and $\mathrm{Na_2S}$, which are then tested in the extract. Options (a), (c) and (d) are these fusion reactions. The reaction of CuO with C is the test for carbon (and hydrogen) by oxidation. It is not part of Lassaigne's test.

(d) A-III, B-IV, C-I, D-II

$\mathrm{Co^{2+}}$ is in Group IV (with $\mathrm{Ni^{2+}}$, $\mathrm{Zn^{2+}}$, $\mathrm{Mn^{2+}}$). $\mathrm{Mg^{2+}}$ is in Group VI. $\mathrm{Pb^{2+}}$ is in Group I (as $\mathrm{PbCl_2}$, with dilute HCl). $\mathrm{Al^{3+}}$ is in Group III (as $\mathrm{Al(OH)_3}$).