Chemistry Questions and Answers

Answer: B

The mass number is the sum of the protons and neutrons in the nucleus, together called the nucleons.

It is always a whole number and is at least equal to the atomic number, which counts protons only. Atomic weight, by contrast, is an average over the natural isotopes and so is usually fractional.

Answer: B

Copper was the first metal used by man.

It occurs native and is soft enough to hammer cold, so it was worked long before smelting was understood; the Chalcolithic or Copper Age is named for it. Bronze is an alloy of copper and tin and came later, and iron later still because it needs much higher temperatures to smelt.

Answer: D

Hydrogen is called the fuel of the future.

It has the highest energy content per unit mass of any fuel, about \(142\ \mathrm{kJ\,g^{-1}}\), and its only combustion product is water: \[\mathrm{2H_2 + O_2 \rightarrow 2H_2O}\] The obstacles are producing it without fossil fuels and storing a gas of such low density safely.

Answer: D

Boiling cannot remove permanent hardness.

Boiling decomposes the bicarbonates of calcium and magnesium and so removes only temporary hardness. Permanent hardness comes from their chlorides and sulphates, which are unaffected by heat; it is removed by adding washing soda or caustic soda, by ion exchange, or by distillation.

Answer: D

The law of constant proportions was enunciated by Joseph Proust in 1799.

It states that a given compound always contains the same elements in the same proportion by mass, whatever its source or method of preparation - water is always 1 part hydrogen to 8 parts oxygen. Dalton's law of multiple proportions came a few years later.

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.