Chemistry

(b) $\mathrm{Cr_{2}O_{3}}$

Chromium(III) oxide gives glass a green colour. $\mathrm{Fe_2O_3}$ gives brown or yellow, PbO gives a heavy lustrous glass, and ZnO gives opacity or a white appearance.

(b) Sulphide

Froth floatation is used for sulphide ores such as galena (PbS), zinc blende (ZnS) and copper pyrites. The sulphide particles are wetted by oil and rise with the froth, while the gangue is wetted by water and sinks.

(b) ZnO

Zinc oxide is called philosopher's wool (also "nil album") because it forms white, woolly flakes when zinc burns in air: $2Zn + O_2 \rightarrow 2ZnO$. $ZnSO_4.7H_2O$ is white vitriol.

(b) Isomerization

Ammonium cyanate ($NH_4CNO$) changes into urea ($NH_2CONH_2$) on heating (Wohler's synthesis). Both have the same molecular formula $CH_4N_2O$ but different structures, so the change is isomerization.

(c) Lavoisier

Antoine Lavoisier is called the father of modern chemistry for establishing the law of conservation of mass and explaining combustion as reaction with oxygen.

(c) Both a and b

Formalin is a 40% aqueous solution of formaldehyde. It kills microbes, so it is a disinfectant, and it hardens tissue, so it is used to preserve biological specimens.

(a) phenol + formaldehyde

Bakelite is a thermosetting condensation polymer made by heating phenol with formaldehyde, with water eliminated.

(b) Avogadro

Avogadro (1811) introduced the idea of the molecule as the smallest particle of a substance that can exist freely, distinguishing it from the atom.

(b) Rutherford

Rutherford's alpha-particle scattering experiment (1911) showed that an atom has a small, dense, positively charged nucleus.

(c) Lead

Galena is lead sulphide, $PbS$, the chief ore of lead.

(b) $\mathrm{(CaSO_{4})_{2}.H_{2}O}$

Plaster of Paris is calcium sulphate hemihydrate, $CaSO_4.\tfrac{1}{2}H_2O$, written as $(CaSO_4)_2.H_2O$. It is made by heating gypsum, $CaSO_4.2H_2O$, at about $120^\circ C$.

(b) $\mathrm{FeSO_{4}.7H_{2}O}$

Green vitriol is hydrated ferrous sulphate. Blue vitriol is $CuSO_4.5H_2O$ and white vitriol is $ZnSO_4.7H_2O$.

(d) Benzene is natural polymer

Ethyne ($HC\equiv CH$) is unsaturated and fits $C_nH_{2n-2}$ with $n=2$. Three ethyne molecules combine to give benzene, $3C_2H_2 \rightarrow C_6H_6$, on passing through a red-hot tube. Benzene itself is a single small molecule, not a natural polymer, so (d) is the false statement.

(c) Iron

Haematite is $Fe_2O_3$, a major ore of iron.

(c) Hydrogen

Hydrogen burns in air with a pale blue flame to form water, and palladium adsorbs (occludes) large volumes of it, up to about 900 times its own volume.

(b) $\mathrm{O_{2}}$

On heating, potassium nitrate gives potassium nitrite and oxygen: $2KNO_3 \rightarrow 2KNO_2 + O_2$. Oxygen is absorbed by alkaline pyrogallol solution.

(c) Insecticides

BHC (benzene hexachloride, $C_6H_6Cl_6$, also called gammexane or lindane) is used to kill insects.

(a) Antiseptic

Tincture of iodine is a solution of iodine in alcohol, applied to wounds to prevent infection. It does not relieve pain, so it is not an analgesic.

(a) Ca(OCl)Cl

Bleaching powder is calcium chlorohypochlorite, $Ca(OCl)Cl$ (commonly written $CaOCl_2$), made by passing chlorine over slaked lime: $Ca(OH)_2 + Cl_2 \rightarrow CaOCl_2 + H_2O$.

(c) $\mathrm{AgNO_{3}}$

Silver nitrate is called lunar caustic because it burns (cauterises) skin and silver was associated with the moon.

(b) Lead

Litharge is lead(II) oxide, $PbO$, a compound of lead obtained when lead is heated in air.

(b) 5

A 5% solution by mass has 5 g of solute in every 100 g of solution:

$\text{mass of NaOH}=\frac{5}{100}\times 100 = 5\ \text{g}$

(a) 66

$CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2$

Molar mass of $CaCO_3 = 100$ g/mol, so $150\ \text{g} = 1.5$ mol. One mole gives one mole of $CO_2$ (44 g), so

$m(CO_2) = 1.5 \times 44 = 66\ \text{g}$

(d) Air

Copper sulphate, carbon dioxide and washing soda are compounds with fixed composition. Air is a mixture of nitrogen, oxygen and other gases in variable proportion.

(b) basic

Metal oxides are generally basic oxides, and those that dissolve in water form hydroxides, e.g. $Na_2O + H_2O \rightarrow 2NaOH$ and $CaO + H_2O \rightarrow Ca(OH)_2$.

(b) Hydrogen

In dilute NaCl solution, water is reduced at the cathode more easily than $Na^+$: $2H_2O + 2e^- \rightarrow H_2 + 2OH^-$. Chlorine forms at the anode in concentrated solution (oxygen in very dilute solution).

(c) $\mathrm{C_{3}H_{6}O_{3}}$

Taking 100 g: C $= 40/12 = 3.33$, H $= 6.67/1 = 6.67$, O $= 53.33/16 = 3.33$ mol. Dividing by 3.33 gives the ratio $1:2:1$, so the empirical formula is $CH_2O$ (mass 30).

$n = \frac{90}{30} = 3$

The molecular formula is $C_3H_6O_3$.

(b) 5

Molar mass of $\mathrm{Ca(OH)_2} = 40 + 2(16+1) = 74\ \mathrm{g\,mol^{-1}}$, so 370 g is $370/74 = 5$ mol.

$\mathrm{CaO + H_2O \rightarrow Ca(OH)_2}$

The ratio is 1 : 1, so 5 mol of CaO is needed.

(c) $\mathrm{C_6H_{12}O_6}$ (the intended option)

Empirical formula mass of $\mathrm{CH_2O} = 12 + 2 + 16 = 30$.

$n = \frac{\text{molecular mass}}{\text{empirical mass}}$

With the molecular weight printed as 90, $n = 3$ and the formula is $\mathrm{C_3H_6O_3}$, which is not among the options. The options fit a molecular weight of 180, where $n = 180/30 = 6$ and the molecular formula is $\mathrm{C_6H_{12}O_6}$ (glucose).

(d) $\mathrm{CHCl_3}$

The simplest ratio C : H : Cl = 1 : 1 : 3 gives the empirical formula $\mathrm{CHCl_3}$.

(d) conversion of white phosphorus to red phosphorus

This is a change between two allotropes of the same element. No new substance is formed, only the arrangement of P atoms changes, so it is treated as a physical change. Burning S or C and corrosion of a metal form new substances (oxides), so they are chemical changes.

(c) 106.8

Solubility 178 means 178 g of NaI saturates 100 g of water.

$\text{NaI for 60 g water} = \frac{178}{100}\times 60 = 106.8\ \mathrm{g}$

(a) 23

Fe has atomic number 26, so the neutral atom has 26 electrons. $\mathrm{Fe^{3+}}$ has lost 3 electrons: $26 - 3 = 23$.

(d) 7.5 gram

At 80°C, solubility 100 means 200 g of saturated solution holds 100 g salt and 100 g water. So 20 g of solution holds 10 g salt and 10 g water.

At 30°C, 10 g water dissolves only $\dfrac{25}{100}\times 10 = 2.5$ g salt.

$\text{Crystallised} = 10 - 2.5 = 7.5\ \mathrm{g}$

(b) $\mathrm{N_2}$

$\mathrm{NH_4NO_2 \xrightarrow{\Delta} N_2 + 2H_2O}$

This is the laboratory method for preparing nitrogen.

(d) NaCl

Group I (Na) has one valence electron and Group VII (Cl) needs one, so Na transfers an electron to Cl and forms the ionic compound $\mathrm{Na^+Cl^-}$. $\mathrm{CaCl_2}$ is Group II + VII, $\mathrm{AlCl_3}$ is Group III + VII, and $\mathrm{CCl_4}$ is covalent.

(d) filtration

AgCl is insoluble in water, so it exists as a white precipitate in the solution. An insoluble solid is separated from a liquid by filtration.

(a) $6.023\times10^{23}$

$n(\mathrm{CH_4}) = \frac{1.6}{16} = 0.1\ \mathrm{mol}$

Molecules $= 0.1 \times 6.023\times10^{23} = 6.023\times10^{22}$. Each $\mathrm{CH_4}$ has $6 + 4 = 10$ electrons, so

$\text{electrons} = 6.023\times10^{22}\times 10 = 6.023\times10^{23}$

(c) Cs

Caesium has the lowest ionisation energy of the stable alkali metals, so light easily ejects its electrons (photoelectric effect). It is therefore used in photoelectric cells. Fr is radioactive and rare.

(d) $55.55\,N_A$

1 litre of water has mass 1000 g, so

$n = \frac{1000}{18} = 55.55\ \mathrm{mol}$

Number of molecules $= 55.55\,N_A$.

(a) 24 gm of C(12)

Number of atoms is proportional to moles:

• C: $24/12 = 2$ mol
• Fe: $56/56 = 1$ mol
• Al: $26/27 = 0.96$ mol
• Ag: $108/108 = 1$ mol

24 g of carbon has the most moles, hence the most atoms.

(b) ${}^{0}_{+1}e$

A positron has the same mass as an electron but charge $+1$, so it is written ${}^{0}_{+1}e$. The electron is ${}^{0}_{-1}e$.

(a) $[\mathrm{Ar}]\,3d^{5}4s^{1}$

Cr (Z = 24) is an exception to the Aufbau order. A half-filled $3d^5$ subshell with a half-filled $4s^1$ is more stable (symmetry and exchange energy) than $3d^4 4s^2$.

(a) Li and Mg

Li (period 2, group 1) and Mg (period 3, group 2) are diagonal neighbours. They have similar properties because their charge/size ratios are similar, for example both form nitrides and their carbonates decompose on heating.

(d) halogens

The configuration $ns^2np^5$ has seven valence electrons, which is group 17, the halogens (F, Cl, Br, I).

(c) $\mathrm{I^+<I<I^-}$

Removing an electron gives a smaller particle because the same nuclear charge holds fewer electrons, so $\mathrm{I^+}$ is smaller than I. Adding an electron raises electron repulsion, so $\mathrm{I^-}$ is larger than I.

(b) CO

CO binds to haemoglobin about 200-250 times more strongly than $\mathrm{O_2}$, forming carboxyhaemoglobin, which is why CO is poisonous.

(c) butane + isobutene

LPG is mainly butane (with isobutane/isobutene, the option given) and some propane. It is liquefied under moderate pressure and used as cooking fuel. Methane and ethane need very high pressure to liquefy, so they are not its main constituents.

(a) coal gas

Coal gas contains about 50% hydrogen, which is separated from it by liquefying the other gases (CO, $\mathrm{CH_4}$) and fractionating. Producer gas is mainly $\mathrm{N_2}$ and CO, and marsh gas is methane.

(d) $\mathrm{N_2O_5}$

An acid anhydride is the oxide that gives the acid with water:

$\mathrm{N_2O_5 + H_2O \rightarrow 2HNO_3}$

(b) chloropicrin

Tear gas is chloropicrin ($\mathrm{CCl_3NO_2}$, nitrochloroform), which makes the eyes water. Aspirin is acetyl salicylic acid, and CO is a poisonous gas, not a lachrymator.