Topic 6 - Quantitive Chemistry

Question 1

How do you work out the relative atomic mass?

Answer 1

Use the bigger number for the atom on the periodic table.

The mass number

Question 2

How do you work out relative formula mass?

Answer 2

Add up relative atomic mass numbers

E.g. H2O > 1+1+16=18

Question 3

If Magnesium’s atomic mass is 24 and Chlorines is 35.5 what is the relative atomic mass for MgCl₂ ?

Answer 3

24 + 35.5 + 35.5 = 95

Question 4

Using the data that C=12 H=1 O=16, what is the relative atomic mass for C₂H₄(OH)₂ ?

Answer 4

• 2 x 12 = 24
• 1 x 4 = 4
• (16 + 1) x 2 = 34
• 34+ 4 + 24 = 64

Question 5

What is the formula to work out the percentage mass of an element in a compound?

Answer 5

Atomic mass x No. of atoms/ relative atomic mass of whole compound

Question 6

What is the percentage of sodium in sodium carbonate Na₂CO₃ when Na = 23, C = 12 and O = 16?

Answer 6

23 x 2 / (23x2 + 12 + 16x3)

23 x 2 / 106

46 / 106 = 43.4%

Question 7

How do you work out Empirical formulas?

Answer 7

1. List all the elements in the compound
2. underneath them write their experimental masses
3. Divide each mass by the atomic mass for that element
4. Turn answers you get into a ratio
5. When the ratio is in its simpliest form this tells you the empirical formula

Question 8

Find the empirical formula for magnesium oxide produced when 9.6g of magnesium react with 6.4g of oxygen. (magnesium = 24 oxygen = 16)

Answer 8

1. Mg & O
2. 9.6 6.4
3. 9.6/24=0.4 6.4/16=0.4
4. (Put into ratio) 0.4:0.4 = 1:1
5. 1 Mg to 1 O = MgO

Question 9

What is the three step way to calculating masses in reactions?

Answer 9

1. Write out the balanced equations
2. work out the relative formula mass (only for the bits you need)
3. Apply the rule: divide to get one then multipply to get all

Question 10

What mass of magnesium oxide is produced when 60g of magnesium is burned in air?

Answer 10

1. _{(Write out the balnaced equation)} 2Mg + O₂ > 2MgO
2. _{(Work out the relative formula masses)} 2x24 > 2 x (24 + 16) = 48 > 80

• This tells us that 48g of Mg reacts to give 80g of MgO
  3. _{(Divide to get one, then mulitply to get all)}
• 48g = 80g
• 1g = (80 / 48) 1.67g
• 60g = (1.67 x 60) 100g
• This shows 60g of Mg will produce 100g of MgO

Question 11

What is the formula for Percentage yield?

Answer 11

actual yield (g) / theoretical yield (g) x 100

Question 12

Why is the actual yield usually less than the theoretical yield?

Answer 12

• Incomplete reactions
• losses during the experiment
• unwanted reactions

Question 13

What is the yield of a reaction?

Answer 13

Is the mass of product obtained in the reaction

Question 14

Why does waste in reactions cost money ?

Answer 14

• The waste isn’t commercially useful
• Disposing of harmful wastes safely is very expensive because they can pose a threat to people and the environemnt

Question 15

What characteristics are chemists looking for in reactions so less waste is produced?

Answer 15

1. A high percentage yield - lots of product is made from raw materials
2. All of the products are commercially useful
3. They are a suitable speed - so the products are made quickly and safely