PowerPoint Presentation
S.L.O.P - Paper 1
Unit 3 Quantitative Chemistry
Number of elements in a formula Example: FeBr3
The number of elements is the same as the number of symbols.
Each different symbol starts with a capital letter.
FeBr3 So we have two different elements Fe = Iron, Br = Bromine
Task 1: How many different elements are in each chemical formula?
a) Fe2O3 b) CaCO3 c) H2 d) AgNO3 e) H2SO4
Number of atoms of each element in a formula Example:
Na2CO3The small numbers on the bottom right of each symbol tell you how many atoms of that element you have. If there is no
number to the right you have one atom of that element.
Task 2: How many atoms of each element are in these formulae?
Also give the total number of atoms:
a) FeCl3 b) H2SO4 c) AgNO3 d) K2SO4 e) CH4 d) NH3 e) SiO2
Na2CO3
2 atoms of Na
(sodium) 1 atom of C
(ca
on)
3 atoms of O
(oxygen)
= 6 atoms in total
Periodic table on back!
Name: Class:
Revision 1.0: © L. Tull 2019 1
Use of
ackets in formulae Example:
Al2(SO4)3The number of atoms of each element inside the
ackets is multiplied by the number on the bottom right outside the
ackets.
Task 3: How many atoms of each element are in these formulae?
Also give the total number of atoms:
a) Mg(NO3)2 b) Ca(OH)2 c) Fe(C2O4)3 d) Al2(CO3)3
Al2(SO4)3
2 atoms of Al
(aluminium) 1 atom of S
(sulfur) inside the
ackets, however
multiply by 3 as that’s the number
outside: 1 x 3 = 3 atoms of S
4 atoms of O
(oxygen) inside the
ackets,
however multiply by 3 as
that’s the number outside:
4 x 3 = 12 atoms of O
= 17 atoms in total
Balancing equations Example: 4 Al + 3 O2→ 2 Al2O3
The number of atoms of each element either side of the a
ow should be the same.
__ Al + __ O2 → __ Al2O31. Draw a line down from
the a
ow in the
unbalanced equation and
list the symbols of all
elements either side.
Al =
O =
Al =
O =
__ Al + __ O2 → __ Al2O3
Al =
O =
Al =
O =
2. Count the number of
atoms of each element
cu
ently in the equation.
1
2 3
2
Continued on next page… 2
__ Al + __ O2 → __ Al2O3
Al =
O =
Al =
O =
3. Work on one element (row)
at a time, finding a number
that both numbers fit into. e.g
oxygen’s original 2 and 3 both
fit into 6. Put the number you
multiplied by into the co
ect
gap in the equation. If at the
end it didn’t need multiplying,
just put a 1 in the gap or leave
it blank.
1
2 3
2
Balancing equations continued…
x 3 = 6 x 2 = 6
3 2
4. Note how when I multiplied
Al2O3 by 2 to get 6 oxygens, I
also multiplied the Al2 bit of it
y 2. Track these changes as
you go!
__ Al + __ O2 → __ Al2O3
Al =
O =
Al =
O =
1
2 3
2
x 3 = 6 x 2 = 6
3 2
x 2 = 4
__ Al + __ O2 → __ Al2O3
Al =
O =
Al =
O =
1
2 3
2
x 3 = 6 x 2 = 6
3 2
x 2 = 4x 4 = 4
45. Now we can check that the
numbers of atoms of each
element are the same.
Al = 4 Al = 4
O = 6 O = 6
The equation is balanced.
Now apply what you’ve learned to
the questions on the next page.
3
Task 4: Balance these equations:
a) ___ Fe + ____ Cl2→ ___ FeCl3
) ___ Mg + ___ HCl → ___ H2 + ___ MgCl2
c) ___ N2 + ___ O2 → ___ NO
d) ___ N2 + ___ O2 → ___ NO2
e) ___ Fe2O3 + ____ C → ____ CO2 + ___ Fe
f) ___ Na + ___ H2O → 2 NaOH + ___ H2
4
Calculating Relative Formula Mass (Mr) Example:
FeCl3
FeCl3
1 atom of iron
1 x 56 = 56
Look for the elements’ symbols on the periodic table, you
need to use the mass numbers (They’re the massive ones!).
Note: Sometimes they give you the mass numbers of each element in the Q.
3 atoms of chlorine
3 x 35.5 = 106.5
XXXXXXXXXX = Mr = 162.5 g/mol
You need to add up the masses of each atom of each element.
Don’t forget the unit of
“grams per mole” (g/mol).
Example:
Al2(SO4)3
Al2(SO4)3
This example has
ackets, remember what we looked at
earlier, multiple the number inside the
ackets by the
number on the outside to get the full number of atoms of
that element.
2 atoms of Aluminium
2 x 27 = 54
3 atoms of Sulfu
3 x 32 = 96 12 atoms of Oxygen
12 x 16 = 192
XXXXXXXXXX = Mr = 342 g/mol
Now apply what you’ve learned to the questions on the next page.
5
Task 5: Calculate the Relative Formula Mass (Mr) of each substance:
Note: Use your copy of the periodic table to find the relative atomic mass:
a) FeCl3
) H2SO4
c) AgNO3
d) K2SO4
e) CH4
f) NH3
g) SiO2
h) Mg(NO3)2
i) Ca(OH)2
j) Fe(C2O4)3
k) Al2(CO3)3
6
Calculating Elemental Percentage Example:
Al2O3Sometimes you will be asked a question like this:“What is the percentage by mass in Al2O3 , of aluminium?
Step 1: Calculate M
Step 2: Divide the total mass of the particular element by the Mr then x 100
Al2O3
2 atoms of Aluminium
2 x 27 = 54
3 atoms of Oxygen
3 x 16 = 48
54 + 48 = Mr = 102 g/mol
Aluminium made up 54 out of the total of 102 so:
54
102
x 100 = 52.9 %
Task 6: Calculate the percentage by mass for the specified element,
ound to 1 d.p:
a) FeCl3 %Fe?
) H2SO4 %O?
c) Ca(OH)2 %H?
d) Al2(CO3)3 %C?
7
The Mole
Chemical amounts are measured in moles, the symbol for the unit mole is mol.
The mass of one mole of a substance is equal to its relative formula mass (Mr) in
grams
e.g. 1 mol of FeCl3 (Mr = 162.5 g/mol) = 162.5 g
This is why the units of relative formula mass are “grams per mole” – It’s how
many
grams 1 mole would weigh! So two moles of a substance would be twice its Mr….
e.g. 2 mol of FeCl3 (Mr = 162.5 g/mol) = 325g
This gives us the equation: Mass = Mr x mol
Real world
mass in
grams (g)
Relative formula
mass in grams
per mole
(g/mol)
How many
moles of the
substance you
have (mol)
This can be rea
anged to give:
mass
M
mol =
mass
mol
Mr =
The mass in this equation needs to be in grams, but modern papers are giving you
masses in:
• nanograms (ng) n x 10-9
• micrograms (μg) n x 10-6
• milligrams (mg) n x 10-3
kilograms (kg) n x 103
• tonnes (T/Mg) (megagrams) n x 106.
Whatever number they give you (if not in grams), just multiply by the co
ect power
as above and slot into the equation → remembering these is easier than
emembering conversions!
8
The Mole examples…..
Example 1:
What is the mass of 3.5 moles of Methane (CH4)?, Mr = 16 g/mol
Mass = Mr x mol
Mass = 16 x 3.5 = 56 g
Example 2:
How many moles are in 348 g of aluminium oxide (Al2O3)? Mr = 102 g/mol
Mol = mass / M
Mol = 348 / 102 = 3.41 mol
Example 3:
What is the Mr of two moles of a compound that weighs 204 g?
Mr = mass/mol
Mol = 204 / 2 = 102 g/mol
Therefore it must be aluminium oxide…
Example 4:
How many moles are in 5 kg of ca
on dioxide (CO2)? Mr = 44 g/mol
Mol = mass / M
Mol = 5 x 103 / 44 = 113.6 mol
Example 5:
How many moles in 600 mg of paracetamol? (C8H9NO2) Mr = 151 g/mol
Mol = mass / M
Mol = 600 x 10-3 / 151 = 3.97 x 10-3 mol
Make sure you know the powers of ten that each prefix stands for, this will
save you time in your biology, physics and chemistry papers!
9
Task 7: Using the mole equation:
a) Write the mole equation in its basic form:
) Write the mole equation rea
anged to make mol the subject:
c) Write the mole equation rea
anged the make Mr the subject:
d) Calculate the number of moles in:
i) 50g of Calcium Mr (Ca) = 40g/mol
ii) 100 g of Zinc Mr (Zn) = 65g/mol
iii) 300 tonnes of Iron chloride, Mr (FeCl3) = 162.5 g/mol
e) Calculate the mass of:
i) 0.5 moles of Cobalt, Mr (Co) = 59 g/mol
ii) 0.75 moles of Methane, Mr (CH4) = 16 g/mol
iii) 12 moles of Zinc chloride, Mr (ZnCl2) = 136 g/mol
10
Task 8: Moles challenge – For levels 5 and higher:
How many moles of each substance are there? You will first need to calculate Mr in
order to use the equation mol = mass / M
a) 150g of Bromine – Br2
) 540g of calcium chloride – CaCl2
c) 460g of Iron (II) hydroxide – Fe(OH)2
What is the mass of each substance? You will first need to calculate Mr in order to
calculate mass using the equation mass = Mr x mol
d) 3 moles of Vanadium, V
e) 0.75 moles of Lead oxide Pb2O
f) 4 moles of Aluminium chloride AlCl3
Calculate the Mr of the unknowns and attempt to identify them by writing a
formula that matches the Mr (there are multiple solutions for each).
g) 4 moles of X, has a mass of 68 g, what is the Mr of X and suggest its identify by
writing a formula that matches the Mr you calculated.
h) 5.4 moles of Y has a mass of 162g, Calculate the Mr of Y and suggest its identity
y writing a formula that matches the Mr you calculated.
11
Avogadro’s Constant
The real world masses of atoms doesn’t match our metric system.
However the mass = Mr x mol equation makes it so that 1 mole of
a substance weights exactly its Mr in grams.
This is because the mole is based on a number called Avogadro’s constant
which translates the mass of atoms to the scale of our