Chapter 9 Moles

Question 1

What is Relative Mass?

Answer 1

Ar

It is a standardised mass of an atom which is compared to the Carbon -12 or 12C which is given a mass of exactly 12.

Question 2

What is Carbon -12

Answer 2

The isotope of Carbon that has a mass number of 12.
This is the standard from which all other relative masses are calculated.

Eg 12 H atoms are equal to the weight of 1C atom so the mass of H is equal to 1.008 atomic mass units.

Question 3

What is the difference between Relative MOLECULAR Mass and Relative FORMULA Mass?

Answer 3

Both are calculated from the sum of the relative atomic masses of the constituent elements.

Relative molecular mass is the mass of a molecule relative to the mass of 12C; Symbol Mr

• used for molecules
• N2, O2 and CO2

Relative formula mass is the mass of a formula unit relative to the mass of 12C.

• used for ionic compunds and covalent network substances.
• NaCL or MgO.

Question 4

What is a Mole?

Answer 4

Represented by a ‘n’. Unit mol.

It is the amount of substance that contains the same number of fundamental particles as there are in atoms in 12g of carbon-12.

Mole is word often referred to as a counting measure to the ‘amount of substance’. Eg like a dozen.

Question 5

What is molar Mass?

Answer 5

The mass of 1 mol of a substance measured in g mol -1; symbol m

Units gmol-1

Question 6

What is Avogadro’s number or constant?

Answer 6

The number of particles in a mole; Symbol Na = 6.022 x 10^23 gmol -1

Question 7

What is a formula

Answer 7

A representation of an element or compound using symbols for its constituent elements. It shows the ratio of atoms in a compound or the number of atoms in a molecule.

Question 8

Calculations using the mole and Avogadro’s number.

Answer 8

particles
n= ———————
6.022 x 10^23

• mole m which is given the symbol ‘n’ and the unit mol
• Avogadro’s number, which is given the symbol Na and has the value 6.022x10^23
• the actual number of particles (atoms, ions or molecules) which is given the symbol N

Question 9

How to calculate mole?

Answer 9

mass in g
n= ———————-
Molar mass in gmol-1

Question 10

What is % composition

Answer 10

The percentage composition of a compound tells you the proportion by mass of the different elements in that compound . The proportion of each element expressed as a percentage of the total mass of the compound.

Question 11

How to calculate percentage compostion

Answer 11

mass of element in 1mol of the compound

% by mass of an element in a compound = ————————x 100
Molar mass of compound