Everything Science Grade 11

CHAPTER 6. QUANTITATIVE ASPECTS OF CHEMICAL CHANGE 6.2

QUESTION

Calculate the number of moles of iron (Fe) in a111.7 g sample.

SOLUTION

Step 1 : Find the molar mass ofiron

If we look at the periodic table, we see that the molar mass of iron is55.85

g.mol−^1. This means that 1 mole of iron will have a mass of 55.85 g.

Step 2 : Use the molar mass and sample mass to calculate the number of moles of iron

If 1 mole of iron has amass of 55.85 g, then:the number of moles ofiron in

111.7 g must be:

111. 7 g

55. 85 g· mol−^1

= 2mol

There are 2 moles of iron in the sample.

Example 2: Calculating mass frommoles

QUESTION

You have a sample thatcontains 5 moles of zinc.

1. What is the mass of the zinc in the sample?


2. How many atoms ofzinc are in the sample?

SOLUTION

Step 1 : Find the molar mass ofzinc

Molar mass of zinc is 65. 38 g· mol−^1 , meaning that 1 mole of zinc has a mass

of 65.38 g.

Step 2 : Calculate the mass of zinc, using moles and molar mass.

If 1 mole of zinc has a mass of 65.38 g, then 5 moles of zinc has a mass of:

65. 38 g× 5 mol = 326. 9 g (answer to a)

Step 3 : Use the number of moles of zinc and Avogadro’s number to calculate the

number of zinc atoms in the sample.

5 × 6. 022 × 1023 = 30. 115 × 1023

(answer to b)