How do you work out the relative atomic mass?
Use the bigger number for the atom on the periodic table.
The mass number
How do you work out relative formula mass?
Add up relative atomic mass numbers
E.g. H2O > 1+1+16=18
If Magnesium’s atomic mass is 24 and Chlorines is 35.5 what is the relative atomic mass for MgCl2 ?
24 + 35.5 + 35.5 = 95
Using the data that C=12 H=1 O=16, what is the relative atomic mass for C2H4(OH)2 ?
- 2 x 12 = 24
- 1 x 4 = 4
- (16 + 1) x 2 = 34
- 34+ 4 + 24 = 64
What is the formula to work out the percentage mass of an element in a compound?
Atomic mass x No. of atoms/ relative atomic mass of whole compound
What is the percentage of sodium in sodium carbonate Na2CO3 when Na = 23, C = 12 and O = 16?
23 x 2 / (23x2 + 12 + 16x3)
23 x 2 / 106
46 / 106 = 43.4%
How do you work out Empirical formulas?
- List all the elements in the compound
- underneath them write their experimental masses
- Divide each mass by the atomic mass for that element
- Turn answers you get into a ratio
- When the ratio is in its simpliest form this tells you the empirical formula
Find the empirical formula for magnesium oxide produced when 9.6g of magnesium react with 6.4g of oxygen. (magnesium = 24 oxygen = 16)
- Mg & O
- 9.6 6.4
- 9.6/24=0.4 6.4/16=0.4
- (Put into ratio) 0.4:0.4 = 1:1
- 1 Mg to 1 O = MgO
What is the three step way to calculating masses in reactions?
- Write out the balanced equations
- work out the relative formula mass (only for the bits you need)
- Apply the rule: divide to get one then multipply to get all
What mass of magnesium oxide is produced when 60g of magnesium is burned in air?
- (Write out the balnaced equation) 2Mg + O2 > 2MgO
- (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
What is the formula for Percentage yield?
actual yield (g) / theoretical yield (g) x 100
Why is the actual yield usually less than the theoretical yield?
- Incomplete reactions
- losses during the experiment
- unwanted reactions
What is the yield of a reaction?
Is the mass of product obtained in the reaction
Why does waste in reactions cost money ?
- 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
What characteristics are chemists looking for in reactions so less waste is produced?
- A high percentage yield - lots of product is made from raw materials
- All of the products are commercially useful
- They are a suitable speed - so the products are made quickly and safely
