Everything Science Grade 11

6.3 CHAPTER 6. QUANTITATIVE ASPECTS OF CHEMICAL CHANGE

QUESTION

You are given a 5 mol sample of sodium. What mass of sodium is in the sample?

SOLUTION

Step 1 : Write the equation to calculate the sample mass.

m = n× M

Step 2 : Substitute values into the equation.

MNa= 22. 99 g· mol−^1

Therefore,

m = 5× 22 .99 = 114. 95 g

The sample of sodium has a mass of 114.95 g.

Example 5: Calculating atoms frommass

QUESTION

Calculate the number of atoms there are in a sample of aluminium thatweighs 80.94 g.

SOLUTION

Step 1 : Calculate the number of moles of aluminium inthe sample.

n =

m

M

=

80. 94

26. 98

= 3 mol

Step 2 : Use Avogadro’s number to calculate the number of atoms in the sample.

Number of atoms in 3 mol aluminium = 3× 6. 022 × 1023

There are 18. 069 × 1023 aluminium atoms in a sample of 80.94 g.

Exercise 6 - 3

1. Calculate the number of moles in each of thefollowing samples:

(a) 5.6 g of calcium

(b) 0.02 g of manganese