1.3 Chemical Calculations

Question 1

relative atomic mass

Answer 1

the average mass of one atom of the element relative to one-twelfth the mass of an atom of carbon-12

Question 2

relative isotopic mass

Answer 2

the mass of one atom of an isotope relative to one-twelfth the mass of one atom of carbon-12

Question 3

relative formula mass

Answer 3

the total average mass of all the atoms in the formula relative to one-twelfth the mass of an atom of carbon-12

Question 4

what is the mass spectrometer used to find

Answer 4

the relative atomic mass of an element

Question 5

what does the mass spectrometer measure:

Answer 5

• the relative mass of each different isotope of an element
• the relative abundance of each isotope of the element

Question 6

what are the 4 main processes of a mass spectrometer

Answer 6

1. ionization
2. acceleration
3. deflection
4. detection

Question 7

ionisation

Answer 7

The vaporized sample passes into the ionization chamber. The particles in the sample are bombarded with a stream of electrons and some of the collisions knock an electron out of the particles to make positive ions.

Question 8

acceleration

Answer 8

The positive ions are accelerated to a high speed by an electric
field.

Question 9

deflection

Answer 9

Different ions are deflected by the magnetic field by different amounts. The amount of deflection depends on the mass of the
ions and their charge:
* the lighter ions are deflected more than the heavier ones
* ions with two positive charges are deflected more than ones with one positive charge.

These two factors are combined in the mass/charge (m/z) ratio.

Question 10

detection

Answer 10

Only ions with a given m/z ratio make it right through the machine to the ion detector. Electrons are transferred from the detector
plate to the positive ion and this produces a current. The larger the current, the higher the abundance of that isotope. The signal is then
amplified and recorded. When the magnetic field is varied, each ion stream can be brought in turn onto the detector to produce a current which is proportional to the number of ions arriving.

Question 11

Avogadro constant,

Answer 11

6.02 × 10^23

Question 12

term molar mass

Answer 12

mass of one mole of a substance

Question 13

term molecular formula

Answer 13

The actual number of atoms of each element
present in the molecule.

Question 14

term empirical formula

Answer 14

The simplest whole number ratio/proportion of
elements present.

Question 15

in an ideal gas:

Answer 15

• the particles are of a negligible size
• the particles move with rapid, random motion
• collisions between particles are perfectly elastic
• there are no intermolecular forces.

Question 16

ideal gas equation

Answer 16

pV=nRT

Question 17

equation for calculations involving solution

Answer 17

moles = volume (dm–3) × concentration (mol dm–3)