Chemistry Questions and Answers

Answer: C

Conduction electrons are more mobile because they collide less frequently.

An electron moves through the conduction band, which is largely empty, so its mean free path between scattering events is long. A hole moves by successive electrons hopping into it within the crowded valence band, so its effective mass is larger and its progress slower - in silicon, electron mobility is about three times hole mobility.

Answer: A

The formula is \(\mathrm{A_2B_5}\).

In a face-centred cubic cell, corner atoms contribute \(1/8\) each and face-centre atoms \(1/2\) each:

\[n(\mathrm{A}) = 8\times\frac{1}{8} = 1,\qquad n(\mathrm{B}) = 6\times\frac{1}{2} = 3\]

Removing one face-centre B takes away \(1/2\) of an atom: \[n(\mathrm{B}) = 3 - \frac{1}{2} = \frac{5}{2}\]

\[\mathrm{A}:\mathrm{B} = 1:\frac{5}{2} = 2:5 \Rightarrow \mathrm{A_2B_5}\]

Answer: C

The cell stops when the electrode potentials of the two electrodes become equal.

Current flows only while there is a potential difference. As the reaction proceeds the concentrations change, and by the Nernst equation \[E = E^\circ - \frac{0.0591}{n}\log Q\] the two half-cell potentials drift together until \(E_{cell}=0\). At that point the cell has reached equilibrium, and \(Q = K\).

Answer: A

Thymine is not found in RNA.

RNA uses uracil in its place; the four RNA bases are adenine, guanine, cytosine and uracil. DNA uses adenine, guanine, cytosine and thymine. Thymine is simply 5-methyluracil.

Answer: C

The side of the box carries phosphorus trisulphide, along with red phosphorus and powdered glass.

The match head holds an oxidising agent, potassium chlorate, with antimony trisulphide and sulphur. Friction converts a trace of the red phosphorus to white phosphorus, which ignites and sets off the head.

Answer: B

This is the zeroth law of thermodynamics.

It is what makes temperature a meaningful property and makes thermometry possible: the thermometer is body B, and two bodies that agree with it agree with each other. It was numbered zero because it was recognised as fundamental only after the first and second laws had been named.

Answer: A

Citric acid is the organic acid.

It is \(\mathrm{C_6H_8O_7}\), a carbon compound with three carboxyl groups, found in citrus fruit. Nitric, sulphuric and phosphoric acids contain no carbon and are mineral acids.

Answer: C

Rusting is corrosion, and it is a chemical change.

Iron combines with oxygen and water to form hydrated iron(III) oxide, a new substance with different properties from the metal. The change cannot be undone by physical means, which is what marks it as chemical rather than physical.

Answer: A

Quartz glass is amorphous.

It has short-range order but no long-range periodicity, so it has no sharp melting point and its properties are the same in all directions. Chrome alum, silicon carbide and graphite are all crystalline, with regularly repeating lattices.

Answer: C

One mole of sodium atoms is the heaviest, at 23 g.

A mole of anything is \(6.022\times10^{23}\) particles, so the mass is simply the molar mass: water 18 g, ammonia 17 g, sodium 23 g, hydrogen \((\mathrm{H_2})\) 2 g.