Transcript Calculations - Life Learning Cloud

Calculations
What you need to know:
• Relative formula mass
• Empirical formula
• % composition by mass
• Use balanced equations to calculate masses of
reactants and products
• Yields
Quick quiz – molecular mass and
empirical formula
**YOU MAY USE REVISION CARDS FROM PREVIOUS LESSONS**
1. Calculate the molecular mass (Mr) of:
– Sodium sulfate
– Calcium hydroxide
– Magnesium nitrate
2. If 9g of aluminium reacts with 35.5g of chlorine, what is the
empirical formula of the compound formed?
3. A compound contains 16g of sulfur and 24g of oxygen. What
is its empirical formula?
4. A compound contains 44.8g of iron and 19.2g of oxygen.
What is its empirical formula?
% composition by mass
A typical McDonald’s meal consists of a burger (250g)
chips (100g) and a drink (200g)
What is the percentage composition of the meal?
1. Total mass = 250 + 100 + 200 = 550g
2. What % is burger? = (250/550) x 100 = 45.4%
3. What % is chips? = (100/550) x 100 = 18.2%
4. What % is drink? = (200/550) x 100 = 36.4%
We use the same method to work out the
percentage compositions of compounds:
Example: What is the percentage composition of NaOH?
Stage 1: find the relative atomic mass (RAM) numbers of the elements
Na = 23
O = 16
H=1
Stage 2: add these RAM numbers together to get the formula mass:
23 + 16 + 1 = 40
Stage 3: work out each elements percentage composition using their RAM
numbers and the formula mass
Na = 23/40 X 100 =
57.5%
O = 16/40 X 100 =
40%
H = 1/40 X 100 =
2.5%
Stage 4: Add up the individual percentages and check that the total equals 100%
Your turn………
1. Calculate the percentage by mass of nitrogen
in each of the following:
a) Ammonia NH3
b) Ammonium chloride NH4Cl
c) Ammonium sulfate (NH4)2SO4
2. NOW MAKE A REVISION CARD
Calculating the masses of reactants and products
Example: what is the mass of carbon dioxide formed
when 36g of carbon is burnt?
Step 1: write the balanced equation:
C + O2  CO2
Step 2: work out relative masses of required chemicals:
C = 12 CO2 = 44
Step 3: divide both by smallest number to find ratio:
C = 12/12 = 1
CO2 = 44/12 = 3.67 (1g C produces 3.67g of CO2)
Step 4: find mass of CO2 formed when 36g C burnt
1 x 36 = 36g 3.67 x 36 = 132g
ANSWER: 132g OF CARBON DIOXIDE IS FORMED WHEN 36g OF CARBON IS BURNT!
Your turn….
1. What mass of water is formed when 4g of
hydrogen is burnt?
2. What mass of sodium chloride is formed
when 10g of sodium hydroxide reacts with
hydrochloric acid?
3. NOW MAKE A REVISION CARD
% YIELDS