Chemistry Questions and Answers

Answer: C

The nucleus of an ordinary hydrogen atom is a single proton.

Protium is the only nuclide with no neutron at all. This is why the hydrogen ion \(\mathrm{H^+}\) is simply a bare proton, and why it cannot exist free in water.

Answer: D

Mesons are held to bind the nucleons together.

Yukawa proposed in 1935 that the strong nuclear force is carried by an exchange particle of intermediate mass; the pi-meson, or pion, was found in cosmic rays in 1947 and matched his prediction. In modern terms the pion exchange is the residual effect of the colour force between quarks.

Answer: C

Dalton's atomic theory explains the conservation of mass.

Its postulate is that atoms are indivisible and are neither created nor destroyed in a chemical reaction; a reaction only rearranges them. Since the same atoms are present before and after, the total mass cannot change.

Answer: A

There are 5 moles.

\[n = \frac{\text{mass}}{\text{molar mass}} = \frac{140}{28} = 5\ \text{mol}\]

which is \(5 \times 6.022\times10^{23} = 3.01\times10^{24}\) silicon atoms.

Answer: B

The monomer of polythene is ethylene (ethene), \(\mathrm{CH_2{=}CH_2}\).

Under high pressure with a peroxide initiator, or over a Ziegler–Natta catalyst, the double bonds open and the units join end to end in an addition polymerisation. Vinyl chloride is the monomer of PVC.

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.