C2 Important Calculations
Relative molecular masses (Mr) and (Ar) – use periodic tables. It’s always the big number
1. What is the Mr of the following
a. MgO
b. CO2
c. ZnCl
d. FeCl3
e. Al2O3
2. If a metal oxide has a Mr of 56 and a formula of XO, X being the unknown
metal. What is the Ar of the unknown metal and what is its name?
Ar
Name:
3. If a metal chloride has a Mr of 80 and a formula of XCl2, X being the
unknown metal. What is the Ar of the unknown metal and what is its
name?
Ar
Name:
Moles
1. Calculate the number of moles of the following
a. 12g of C
b. 12g of Mg
c. 12g of Na
d. 12g of Pb
Percentage Composition 1. What is the percentage of
a. Magnesium in MgO
b. Carbon in CO2
c. Zinc in ZnCl
d. Iron in FeCl3
e. Aluminium in Al2O3
Number of =
moles of X
Percentage
composition
of X
=
mass of X
Ar or Mr of X
Ar of X x 100
Mr of compound
Maximum yield = (mass of X) x (Mr XY)
of XY given
Ar of X
the mass of X
Maximum (Theoretical) Yield and percentage yield
Percentage = mass made x 100
1. Mg + Cl2
MgCl2
yield
maximum yield
a. What is the Ar of Mg?
b. What is the Mr of MgCl2?
c. If 24g of Mg reacted what would the maximum mass of MgCl2 be?
If we only got 90g what is the percentage yield?
d. If 48g of Mg reacted what would the maximum mass of MgCl2 be?
If we only got 120g what would the percentage yield be?
2. O2 + 2H2
2H2O
a. What is the Mr of O2?
b. What is the Mr of H2O?
c. If 32g of O2 reacted what would the maximum mass of H2O be?
Remember there are 2H2O produced
If we got 32g of water what is the percentage yield?
d. If 12g of O2 reacted what would the maximum mass of H2O be?
If we got 10g of water what is the percentage yield?
Notes and Doodling
Empirical Formula
Step 1 – Calculate the number of moles of each substance
Step 2 – divide each of these numbers by the smallest number to give a
ratio
Step 3 – Write down the numbers by elements to give a formula
1.
A sample of the solvent used in one perfume contained 0.60 g of carbon,
0.15 g of hydrogen and 0.40 g of oxygen.
Relative atomic masses: H = l; C = 12; O = 16.
Calculate the empirical (simplest) formula of the solvent.
You must show all of your working to gain full marks for this question.
(4 Marks)
2.
A compound is called phosgenite.
Analysis of this compound shows that it contains:
76.0g lead (Pb), 13.0g chlorine (Cl), 2.2g carbon (C) and 8.8g oxygen (O)
Calculate the empirical formula of this compound.
To gain full marks you must show all your working.
Relative atomic masses: C = 12 ; O = 16 ; Cl = 35.5 ; Pb = 207
(4 Marks)
Percentage Atom Economy
Percentage atom =
economy
Mr of useful product
total Mr of all reactants
x 100
1. Calcium oxide is made by the thermal decomposition of calcium carbonate
This is shown in the equation below:
CaCO3
CaO + CO2
a. What is the Mr of calcium carbonate (CaCO3)?
b. What is the Mr of calcium oxide (CaO)?
c. What is the percentage atom economy in this reaction?
2. Copper sulfate can be made in the following reaction
Copper oxide + sulfuric acid
CuO + H2SO4
copper sulfate + water
CuSO4
+ H 2O
Calculate the percentage atom economy of the reaction
Now look at some questions from past papers and see if you can understand
everything they are asking.
Try to review this work every week to make sure you still know how to calculate
all the different things they may ask in the exam.