Chemistry Questions and Answers

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.

Answer: A

Bauxite, hydrated aluminium oxide \(\mathrm{Al_2O_3\cdot 2H_2O}\), is the most important ore of aluminium.

It is purified to alumina by the Bayer process and then reduced electrolytically in the Hall–Héroult cell. Haematite and magnetite are iron ores and monazite a thorium ore.

Answer: C

This is Henry's law.

\[p = K_H\, x\] where \(x\) is the mole fraction of the dissolved gas and \(p\) its partial pressure above the liquid. It explains why a fizzy drink releases gas when opened, and why divers ascending too quickly suffer decompression sickness as dissolved nitrogen comes out of the blood.

Answer: B

Ethylene ripens green fruit artificially.

It is the plant's own ripening hormone, and a few parts per million in a closed chamber will start the breakdown of chlorophyll and the conversion of starch to sugar. Because it is a gas, one ripening fruit hastens the ripening of others beside it.

Answer: D

The \(\mathrm{Zn^{2+}}\) ion contains 35 neutrons.

Zinc has \(Z=30\) and a mass number of 65, so \[N = A - Z = 65 - 30 = 35\] Forming the dipositive ion removes two electrons; the nucleus is untouched, so the neutron count is unchanged.

Answer: C

Milk is a poor source of vitamin C.

It is rich in calcium, high-quality protein and lactose, and supplies vitamins A, D and the B group, but contains very little ascorbic acid - and pasteurisation destroys most of what there is. This is why an infant fed only on milk needs a separate source of vitamin C.