Chemistry Questions and Answers

Answer: A

Biogas is mainly methane.

It is typically 55–65 per cent \(\mathrm{CH_4}\) with most of the rest carbon dioxide, produced when methanogenic bacteria digest cattle dung and other organic waste in the absence of air. The methane is what makes it burn.

Answer: B

John Dalton proposed the first atomic theory, in 1808.

He postulated that matter is made of indivisible atoms, that all atoms of an element are identical in mass, and that compounds form when atoms combine in fixed small whole-number ratios. The theory explained the laws of conservation of mass and of definite and multiple proportions at a stroke.

Answer: A

Urea has the highest nitrogen content, about 46 per cent.

\(\mathrm{NH_2CONH_2}\) has a molar mass of 60 with 28 units of nitrogen: \[\frac{28}{60}\times 100 = 46.7\%\] Ammonium nitrate is 35 per cent, ammonium chloride 26 per cent and ammonium sulphate 21 per cent, which is why urea is the commonest nitrogenous fertiliser.

Answer: C

Formic acid is so named because it was first obtained by the distillation of ants.

John Ray distilled red ants in 1671 and isolated the acid, and Latin formica means ant. Ants do secrete it as a defensive spray, but the name records how the compound was first prepared, not what it is used for.

Answer: B

Cations are formed by the loss of electrons.

Removing negative charge leaves a net positive ion, as when \(\mathrm{Na} \rightarrow \mathrm{Na^+} + e^-\). The cation is always smaller than the parent atom, since the unchanged nuclear charge now acts on fewer electrons - and often a whole shell has been removed.

Answer: B

A beta particle is identical to an electron.

It is emitted when a neutron in the nucleus converts to a proton, and has the same mass and charge as an orbital electron; only its origin differs. A positron has the electron's mass but the opposite charge.

Answer: B

Helium has the highest first ionisation energy, about \(2372\ \mathrm{kJ\,mol^{-1}}\) - the highest of any element.

Its two electrons occupy the 1s orbital, closest to the nucleus and with no shielding between them and it. Ionisation energy falls down the group as the outer shell moves further out: Ne 2081, Ar 1521, Xe 1170.

Answer: C

Gold is alloyed with copper for jewellery.

Pure 24-carat gold is too soft to hold a setting, so copper - often with a little silver - is added for hardness. 22-carat gold is 22 parts gold in 24, about 91.7 per cent; a higher proportion of copper gives rose gold.

Answer: B

Minamata disease was caused by mercury.

A chemical plant discharged methylmercury into Minamata Bay in Japan from the 1930s; it accumulated in fish and shellfish, and those who ate them suffered severe neurological damage. The disease was formally identified in 1956.

Answer: D

The silver mirror test is given by acetaldehyde and formic acid - that is, 2 and 4.

Tollens' reagent, ammoniacal silver nitrate, oxidises an aldehyde group and deposits metallic silver on the glass: \[\mathrm{RCHO + 2[Ag(NH_3)_2]^+ + 3OH^- \rightarrow RCOO^- + 2Ag\downarrow + 4NH_3 + 2H_2O}\] Formic acid responds because it contains a \(\mathrm{-CHO}\) group as well as \(\mathrm{-COOH}\). Acetone and benzophenone are ketones and do not react.