Everything Science Grade 10

CHAPTER 19. QUANTITATIVE ASPECTS OF CHEMICAL CHANGE 19.1

Step 1:Convert mass into grams

m= 1kg×1 000 = 1 000g

Step 2:Calculate the molar mass

M(MgCl 2 )= 24, 3 g·mol−^1 + (2× 35 , 45 g·mol−^1 ) = 95, 2 g·mol−^1

Step 3:Find the number of moles

n= 1 000g

95 , 2 g·mol−^1

= 10, 5 mol

There are 10 , 5 moles of magnesium chloride in a 1 kg sample.

Group Discussion: Understanding moles, molecules

and Avogadro’s number

Divide into groups of three and spend about 20 minutes answering the follow-

ing questions together:

1. What are the units of the mole? Hint: Check the definition of the mole.

2. You have a 46 g sample of nitrogen dioxide (NO 2 )

a. How manymolesof NO 2 are there in the sample?

b. How many moles of nitrogen atoms are there in the sample?

c. How many moles of oxygen atoms are there in the sample?

d. How manymoleculesof NO 2 are there in the sample?

e. What is the difference between a mole and a molecule?

3. The exact size ofAvogadro’s numberis sometimes difficult to imagine.

a. Write down Avogadro’s number without using scientific notation.

b. How long would it take to count to Avogadro’s number? You can

assume that you can count two numbers in each second.

Exercise 19 - 4

1. Calculate the molar mass of the following chemical compounds:

Chemistry: Chemical change 341