Chemistry Questions and Answers

Answer: D

Cathode rays have both mass and charge.

They are electrons. They are deflected by electric and magnetic fields, which shows they carry charge, and they turn a light paddle-wheel placed in their path, which shows they carry momentum and therefore mass.

Answer: B

The difference lies at the 2nd carbon of the sugar.

RNA contains ribose, which has a hydroxyl group at C-2; DNA contains 2-deoxyribose, which has only a hydrogen there. That missing \(\mathrm{-OH}\) makes DNA far more resistant to hydrolysis, which is why it, and not RNA, stores heredity.

Answer: C

Carbon tetrachloride was used as a fire extinguisher, under the name pyrene.

Its vapour is much denser than air, so it blankets the fire and cuts off oxygen, and it does not conduct electricity, which made it useful on electrical fires. It is no longer used, because hot \(\mathrm{CCl_4}\) can form phosgene.

Answer: D

Standard electrode potential is measured with a voltmeter, specifically a potentiometer or high-resistance voltmeter.

The electrode is coupled to the standard hydrogen electrode, whose potential is defined as zero, and the cell EMF is read at unit concentration, 298 K and 1 bar. A high resistance is needed so that almost no current flows and the reading is the equilibrium potential.

Answer: B

A particle with 13 protons and 10 electrons is the \(\mathrm{Al^{3+}}\) ion.

Thirteen protons identifies aluminium. Having three fewer electrons than protons gives a charge of \(+3\), and the remaining ten electrons are the neon configuration.

Answer: D

The triple point of water is at 4.58 torr and 0.01 °C.

This is the single set of conditions at which ice, liquid water and water vapour coexist in equilibrium. It is exactly reproducible, which is why it was used to define the kelvin: the triple point was fixed at 273.16 K.

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: D

An alloy is a mixture.

It is a homogeneous mixture of two or more metals, or of a metal with a non-metal, in no fixed proportion - brass may be 60–90 per cent copper. Because the composition can vary, an alloy is not a compound.

Answer: A

The atomic weight, or mass number, is the sum of the neutrons and protons.

These two together are the nucleons and carry almost all the mass of the atom. An electron weighs about 1/1836 of a proton, so electrons contribute nothing appreciable to the mass.

Answer: D

Aluminium is the most abundant metal in the earth's crust, at about 8 per cent by mass.

Iron follows at about 5 per cent, then calcium, sodium, potassium and magnesium. Overall the crust's commonest elements are oxygen (46 per cent) and silicon (28 per cent), neither of which is a metal.