Quantitative Chemistry: Calculating Moles of a Compound (DONE)

Question 1

Calculate the formula mass of this compound
CO↓2

Ar carbon = 12
Ar oxygen = 16

Answer 1

We can see that we have one atom of carbon bonded to two atoms of oxygen. We need to work out the relative formula mass of the compound.

1 x 12 = 12
2 x 16 = 32

32 + 12 = 44

The relative formula mass of CO↓2 is 44 M↓r

Question 2

What’s the formula we use to calculate the number of moles of a compound?

Answer 2

Number of moles = mass (g) / relative formula mass (M↓r)

Question 3

You are given a sample of calcium carbonate (CaCO↓3) with a mass of 300g. Calculate the number of moles of calcium carbonate in the sample.

CaCO↓3

Ar Ca = 40
Ar C = 12
Ar O = 16

Answer 3

We have one atom of calcium and one atom of carbon and three atoms of oxygen. We first calculate the relative formula mass.

1 x 40 = 40
1 x 12 = 12
3 x 16 = 48
48 + 12 + 40 = 100

Number of moles = mass (g) / relative formula mass Mr

300/100 = 3

This means we have 3 moles of calcium carbonate in the sample.

Question 4

You are given 380 grams of magnesium chloride (MgCl↓2). How many moles of magnesium chloride have you been given?

MgCl↓2

Ar Mg = 24
Ar Cl = 35.5

Answer 4

1 x 24 = 24
2 x 35.5 = 71
71 + 24 = 95

Number of moles = mass (g) / relative formula mass (M↓r)

4 = 380/95

We have been given 4 moles of magnesium chloride.

Question 5

You are given a sample of lithium sulphate with a mass of 990g. Calculate the number of moles of lithium sulphate in the sample.

Li↓2SO↓4

Ar Li = 7
Ar S = 32
Ar O = 16

Answer 5

Li =2 x 7 = 14
S = 1 x 32 = 32
O = 4 x 16 = 64
Mr = 110

990/110 = 9

9 moles

Question 6

You have been given 64.5g of beryllium hydroxide Be(OH)↓2. Calculate the number of moles of beryllium hydroxide that you have been given.

Be(OH)↓2

Ar Be = 9
Ar O = 16
Ar H = 1

Answer 6

1 x 9 = 9
2 x 16 = 32
2 x 1 = 2
Mr = 43

64.5 ÷ 43 = 1.5 moles