Chemistry Questions and Answers

Answer: C

Chromic acid also renders iron passive.

Strong oxidising acids build a thin, adherent, impervious film of oxide on the iron surface, and that film prevents further attack - which is why concentrated nitric acid can be carried in steel tankers. Acetic, oxalic and citric acids are weak and non-oxidising.

Answer: C

Iron is not supplied by ordinary fertilisers.

Fertilisers supply the three macronutrients - nitrogen, phosphorus and potassium, the N-P-K of the label - because plants need them in quantity. Iron is a micronutrient, needed in traces and usually present in the soil already; manures supply it when it is lacking.

Answer: D

Phosphorus exists as \(\mathrm{P_4}\).

White phosphorus is a tetrahedral molecule of four atoms with P–P–P angles of 60°. That severe angular strain is why white phosphorus is so reactive that it ignites in air and must be stored under water.

Answer: B

Secondary structure is the alpha helix and the beta pleated sheet.

These are local, regular foldings of the polypeptide backbone, held by hydrogen bonds between the carbonyl oxygen of one residue and the amide hydrogen of another. The primary structure is the amino-acid sequence, the tertiary the overall fold of one chain, and the quaternary the assembly of several chains.

Answer: A

The chemical changes are (a) and (b).

Rusting forms hydrated iron(III) oxide and burning magnesium forms magnesium oxide - new substances, and neither change is reversible by simple physical means. Melting ice and dissolving sugar produce no new substance and are reversed by freezing and by evaporation.

Answer: B

\(\mathrm{IF_7}\) is a pentagonal bipyramid.

Iodine contributes seven bonding pairs and has no lone pair, so by VSEPR the seven pairs arrange themselves as \(sp^3d^3\) - five fluorines in an equatorial plane at 72° and two along the axis. It is one of the few molecules with this geometry.

Answer: D

Graphite rods slow fast neutrons down to thermal energies.

Neutrons released by fission travel at about \(2\times10^{7}\ \mathrm{m\,s^{-1}}\), too fast to be captured efficiently by uranium-235. Colliding elastically with the light carbon nuclei of graphite, they lose energy until they are in thermal equilibrium with the moderator and can sustain the chain reaction. Cadmium or boron control rods, by contrast, absorb neutrons.

Answer: B

The nucleus was discovered by Rutherford, from the Geiger–Marsden experiment of 1909–11.

Alpha particles fired at a thin gold foil mostly passed straight through, but about one in eight thousand was deflected through more than 90°. Rutherford concluded that almost all the mass and all the positive charge is concentrated in a tiny central nucleus.

Answer: B

Nitrogen is the major constituent of air, at about 78 per cent by volume.

Oxygen accounts for about 21 per cent, argon 0.93 per cent and carbon dioxide about 0.04 per cent. Hydrogen is present only in traces, because it is light enough to escape the atmosphere.

Answer: D

One Avogadro number of helium atoms weighs 4.00 g.

By definition, \(N_A=6.022\times10^{23}\) atoms is one mole, and one mole of an element weighs its gram atomic mass. Helium's atomic mass is 4, so \[m = 1\ \text{mol}\times 4\ \mathrm{g\,mol^{-1}} = 4.00\ \mathrm{g}\] Multiplying 4 by \(6.02\times10^{23}\) would give the mass of that many moles, not of that many atoms.