Chapter 4- The Mole

Question 1

Mole

Answer 1

The amount of substance that contains the same number of specified particles as there are atoms in 12g of carbon-12.

• 1 atom of 12C has a relative atomic mass of 12 exactly.
• 1 mol of atoms of 12C has a mass of 12g exactly.

Question 2

Relative Isotopic Mass (Ir)

Answer 2

The relative isotopic mass of an isotope is the mass of an atom of that isotope relative to the mass of an atom of 12C taken as 12 units exactly.

Question 3

Isotopes

Answer 3

Atoms with the same atomic number but different mass numbers.

Question 4

What is the mass spectrometer used for?

Answer 4

• Used to determine the relative masses of different isotopes
• Determines the mass of each isotope, relative to the 12C isotope, and the relative abundances of the isotopes

Question 5

How does the mass spectrometer work?

Answer 5

1) A gas sample enters into the ionisation chamber
2) It is bombarded with electrons, forming ions with a positive charge.
3) The ions are accelerated to high speeds.
4) They enter a magnetic field which causes the ions to move in curved paths.
5) The larger the ion, the less it will curve.

Question 6

Relative Atomic Mass (Ar)

Answer 6

The weighted mean of the relative isotopic masses of an element on the scale where 12C is 12 exactly.

Question 7

Formula for calculating RAM?

Answer 7

Ar = Σ (relative isotopic mass × % abundance) / 100

Question 8

Relative Molecular Mass

Answer 8

• The mass of a molecule relative to the mass of an atom of 12C taken as 12 units exactly.
• It is numerically equal to the sum of the relative atomic masses of the atoms making up the molecule.

Question 9

Relative Formula Mass

Answer 9

• For non-molecular compounds, the term relative formula mass is used (ie NaCl)
• Calculated by taking the sum of the relative atomic masses of the elements in the formula

Question 10

Calculation of number of particles in a substance?

Answer 10

Number of particles = n x NA ,where: n = number of moles and NA = Avogadro’s number (6.02 X 10^23)

Question 11

How many particles does 1 mole contain?

Answer 11

1 mol of particles contains 6.02 × 10^23 particles.

Question 12

Avogadro’s constant

Answer 12

6.02 × 10^23

Question 13

Molar Mass

Answer 13

The mass of 1 mole of a particular element or compound measured in g mol−1.

Question 14

Formula to calculate mass of a given substance?

Answer 14

mass of a given amount of substance (g) = amount of substance (mol) × molar mass (g mol−1)
m=n x M

Question 15

Percentage by mass of an element

Answer 15

The mass of individual elements in a compound expressed as a percentage of the mass of the compound.

Question 16

Formula to calculate percentage by mass of an element

Answer 16

% mass = Ar of element x number of atoms of this element DIVIDED BY molar mass of the compound MULTIPLIED BY 100

Question 17

Empirical Formula

Answer 17

A formula of a compound that shows the elements present and their ratio in a compound

Question 18

Molecular Formula

Answer 18

A formula that represents the actual number of atoms in one molecule of the compound.