2.2 the mole and the avogadro constant

Question 1

What is a mole?

Answer 1

The mole is a way of measuring the amount of a substance in chemistry. It is commonly shortened to ‘mol’.

Question 2

1 mole of any substance contains 6.02x10²³…

Answer 2

atoms / molecules. For example, 1 mole of H₂O contains 6.02x10²³ molecules of H₂O and 1 mole of Fe contains 6.02x10²³ atoms of Fe.

Question 3

We can calculate the number of particles in a substance using…

Answer 3

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

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

The number of moles can be calculated from mass and Mr/Ar.

Answer 5

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

Question 6

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

Answer 6

Number of moles = mass / Ar

number of moles = 23 / 197

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

Question 7

The mole - solutions. The number of moles of a substance in a solution can be calculated from concentration and volume.

Answer 7

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

Question 8

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

Answer 8

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