2 The mole and the Avogadro constant

Question 1

Define a ‘mole’

Answer 1

The amount of a substance that contains the same number of particles as the number of atoms in 12 grams of carbon-12

Question 2

1 mole of any substance contains 6.02x10²³…

Answer 2

atoms / molecules
- 1 mole of H₂O contains 6.02x10²³ molecules of H₂O
- 1 mole of Fe contains 6.02x10²³ atoms of Fe

Question 3

How do we calculate the number of particles?

Answer 3

number of particles = number of moles x Avogadro’s number

- no of particles = n x L

Question 4

(Example) How many particles make up 0.67 moles of ammonia (NH₃)?

Answer 4

Number of particles = 6.02x10²³ x 0.67

number of particles = 4.03x10²³

Question 5

How many atoms are in 57.0g of NH₃?

Answer 5

Mr of NH₃ = 14.0 + (3 x 1.0) = 17.0 g mol^-1

n = m / Mr
n = 57.0 / 17.0 = 3.35 mol

no of particles = n x L
3.35 x 6.022 x 10²³ = 2.02 x 10²⁴

There are 4 atoms present in a molecule of NH₃ so therefore we times 2.02 x 10²⁴ by 4 which equals 8.08 x 10²⁴

Question 6

What is the mass of 1 atom of Carbon?

Answer 6

Ar = 12.0

1 mole of C = 12g
6.022x 10²³ atoms of C = 12.0g

Divide both sides by Avogadro’s number

1 atom of C = 1.9926 x 10^-23 g
3 sig fig = 1.99 x 10^-23 g

Question 7

How can we calculate the number of moles from the mass and Mr?

Answer 7

Number of moles = mass (g) / Mr or Ar

- n = m / Mr

Question 8

(Example) Calculate the number of moles of 23g of gold. Give your answer to 2 significant figures.

Answer 8

Number of moles = mass / Ar

number of moles = 23 / 197

number of moles = 0.12mol (2 s.f)

Question 9

(Example) Which contains the greatest number of atoms?
10.8g of water or 4.8g of helium

Answer 9

H₂O
n = m / Mr
n = 10.8g / 18.0 = 0.6 moles of H₂O
There is 3 atoms present in a H₂O molecule so we therefore times the number of moles by the amount of atoms
0.6 x 3 = 1.8 moles

He
n = m / Ar
n = 4.8 / 4 = 1.2 moles

Water contains the greatest number of atoms.

Question 10

How can we calculate the number of moles in solutions?

Answer 10

Number of moles = concentration (moldm^-3) x volume (dm³)

- n = c x v

Question 11

(Example) Calculate the number of moles of 200cm³ of 0.35moldm^-3 HCl.

Answer 11

Number of moles = concentration x volume

200cm³ must be in dm³: 200 / 1000 = 0.2 dm³

number of moles = 0.35 x 0.2 = 0.07moldm^-3