Chemistry Questions and Answers

Answer: D

Einstein stated that matter can be converted into energy.

His 1905 relation \[E = mc^2\] says that a mass \(m\) is equivalent to an energy \(mc^2\). Because \(c^2\) is about \(9\times10^{16}\ \mathrm{m^2\,s^{-2}}\), even a gram of matter corresponds to some \(9\times10^{13}\) joules - which is why the mass defect in a nucleus releases so much energy.

Answer: A

The answer is \(5x/36\).

The ionisation energy from the ground state is \(x\), so \[E_n = -\frac{x}{n^2}\]

The energy needed to move from \(n=2\) to \(n=3\) is \[\Delta E = E_3 - E_2 = -\frac{x}{9} + \frac{x}{4} = x\left(\frac{1}{4}-\frac{1}{9}\right) = x\cdot\frac{9-4}{36} = \frac{5x}{36}\]

Answer: C

All three are isotopes of hydrogen.

Protium, \(^1_1\mathrm{H}\), has no neutron and makes up 99.98 per cent of natural hydrogen; deuterium, \(^2_1\mathrm{H}\), has one and is stable; tritium, \(^3_1\mathrm{H}\), has two and is radioactive, decaying by beta emission with a half-life of 12.3 years.

Answer: A

The answer is 0.5 mole.

\[n(\mathrm{O}) = \frac{16}{16} = 1\ \text{mol of oxygen atoms}\]

Each \(\mathrm{CO_2}\) molecule contains two oxygen atoms, so \[n(\mathrm{CO_2}) = \frac{1}{2} = 0.5\ \text{mol}\]

Answer: D

Caesium is the most electropositive of the four.

Electropositive character - the tendency to lose an electron - increases down a group as the atom grows larger and the outer electron is more shielded, and decreases across a period. Cs lies lowest in Group 1 among these, so its ionisation energy is the smallest. Francium is more electropositive still but is radioactive and vanishingly rare.

Answer: B

Soda glass is so called because it is made using soda, that is sodium carbonate.

It is a soda-lime-silica glass, melted from sand, sodium carbonate and limestone in roughly the proportions 75:15:10. The soda lowers the melting point of silica from about 1,700 °C to a workable temperature, and the lime keeps the glass from dissolving in water.

Answer: B

Such atoms are isotopes.

They have the same atomic number, so they are the same element, but different mass numbers because the neutron count differs. Carbon-12, carbon-13 and carbon-14 are examples.

Answer: A

Protons and electrons are equal in number in a neutral atom.

The proton carries \(+1.6\times10^{-19}\ \mathrm{C}\) and the electron exactly the opposite, so equal numbers make the atom electrically neutral. The neutron count varies between isotopes and need not match either.

Answer: B

Alum is used in the purification of water.

Potash alum, \(\mathrm{K_2SO_4\cdot Al_2(SO_4)_3\cdot 24H_2O}\), releases \(\mathrm{Al^{3+}}\), whose high charge neutralises the negative charge on suspended clay particles so that they coagulate and settle. It is also used to stop bleeding from shaving cuts, for the same coagulating reason.

Answer: C

Cathode rays are a stream of electrons.

J.J. Thomson showed in 1897 that they are deflected by both electric and magnetic fields towards the positive plate, so they are negatively charged particles, and that their \(e/m\) ratio is the same whatever gas or electrode material is used - proving them a universal constituent of matter.