Chemistry, Stoichiometry, 1.2 The Mole Concept

Define relative atomic mass (Ar)

The weighted average mass of an ATOM, compared to 1/2 the mass of an atom of carbon-12

Define isotope

An atom of the same element with a different number of neutrons

What can abundance of an isotope be measured with?

A mass spectrometer

Define relative molecular (formula) mass (Mr)

The weighted average mass of a COMPOUND compared to 1/2 the mass of an atom of carbon 12

What type of compound does formula mass refer to?

Ionic compounds / formula units

What type of compound does molecular mass refer to?

Molecular covalent compounds

What are moles (n) a measure of?

The amount of particles / elementary entities of a substance (eg. atoms, molecules, ions, electrons etc.)

How many elementary entities does a mole contain?

6.02 x 10^23

What does Avogadro's Law represent?

The relation between amount of a substance and amount of particles it contains

What are the figure and units of Avogadro's Constant (Na / L)?

6.02 x 10^23 mol^ -1

A mole of any substance has the same ... ?

Number of particles

Moles of substances may have different ... ?

Masses / volumes

What does molar mass (M) represent? (Definition and units)

The mass in g of one mole of a substance (g per mol^ -1)

M also = ...

Mr

Explain how to determine the number of different atoms in one mole of a compound

1. Multiply avogadro's constant by the number of moles of the compound there are

2. Multiply the resulting number by the amount of atoms of each element in the compound there are

Determine the number of carbon and oxygen atoms in one mole of CO2

1. (6.02 x 10^23) x 1 = 6.02 x 10^23

2. (6.02 x 10^23) x 1 = 6.02 x 10^23 Carbon atoms in one mole of CO2

(6.02 x 10^23) x 2 = 1.20 x 10^24 Oxygen atoms in one mole of CO2

Determine the total number of atoms in two molecules of benzene (C6H6)

1. (6.02 x 10^23) x 2 = 1.20 x 10^24

2. (1.20 x 10^24) x 12 = 1.45 x 10^25

Determine the molar mass (M) of ethanol (C2H5OH)

Ar(C): 12.01

Ar(H): 1.01

Ar(O): 16

Mr(C2H5OH) = (12.01 x 2) + (1.01 x 6) + 16 = 46.08

M = 46.08 g per mol^ -1

Outline the relationship between mass (g), moles (n), and particle number with formulae including M (molar mass) and L (avogadro's constant)

Mass — (/M) -> Mol

Mol — (xM) -> Mass

Mol — (xL) -> Particle number

Particle number — (/L) -> Mol

Explain how to determine the number of molecules in a certain mass of a substance

1. Divide mass by the Mr of the substance, obtaining number of moles

2. Multiply number of moles by avogadro's constant

How many oxygen molecules are in 16g of O2?

1. 16 / (2 x 16) = 0.5 n

2. 0.5 x (6.02 x 10^23) = 3.01 x 10^23 oxygen particles in 16g of O2

Explain how to determine the mass of a substance with a certain number of particles

1. Divide number of particles by avogadro's constant, obtaining number of moles

2. Times number of moles by M of the substance

Determine the mass (g) of 1.51 x 10^23 carbon atoms.

1. (1.51 x 10^23) / (6.02 x 10^23) = 0.251 n

2. 0.251 x 12.01 = 3.01g of carbon in 1.51 x 10^23 of carbon atoms