Formulae and empirical yield
Year 11
Starter
1.
2.
3.
4.
Magnesium reacts with sulfuric
acid (H2SO4) to form magnesium
sulfate (MgSO4) and hydrogen (H2)
Write out a word equation for this
reaction
Write out a balanced symbol
equation for this reaction
Work out the relative formula
mass for the reactants and
products
What mass of magnesium is
needed to form 40g of
magnesium sulfate?
Element
Symbol
Ar
Magnesium
Mg
24
Sulfur
S
32
Hydrogen
H
1
Oxygen
O
16
Learning objectives
By the end of the lesson you should be able to:
• calculate the percentage yields of reactions
Key words
• percentage yield, yield
How much?
• Many of the substances
we use everyday are
made from chemical
reactions involving other
substances
• If we want 1000kg of C,
what do we need to know
to work out how much of
A and B we need?
• Balanced symbol
equation
• Mr of each substance
Task one
• Complete the worksheet “How much?”
Starter
• Respond to the green pen questions and
comments in your books using RED PEN
Yield
• We can estimate how much product a chemical reaction will
make using RFM
• This is known as the yield
• Reactions do not always produce as much product as they
should. Actual yield is often less. Why?
1. Reactants may not be pure
2. Not all of the reactants combine together
3. Other substances are made that have to be removed as
impurities
4. Some product may be lost during purification
Percentage yield
• We can calculate the percentage yield using the equation:
• Percentage yield = ___actual yield_______
x 100
maximum possible yield
• E.g. titanium metal is made found in the ore titanium dioxide
(TiO2)
• What is the RAM of titanium?
• What is the RFM of titanium dioxide?
• What is the theoretical yield of titanium from 160kg of
titanium dioxide?
• The actual yield is 89kg. Calculate the percentage yield
Calcium oxide (CaO) is made from thermally decomposing
calcium carbonate (CaCO3). Carbon dioxide (CO2) is also
produced.
A company processes 200 tonnes of limestone a day. It
collects 98 tonnes of calcium oxide a day.
What is the percentage yield of the process?
1. Write a word equation for the reaction
2. Write a balanced symbol equation for the reaction
3. Calculate the relative formula masses of the reactant and
products
4. Calculate the maximum amount of calcium oxide 100
tones of calcium carbonate would produce
5. Calculate the percentage yield if 98 tonnes of calcium
oxide are produced
Task two
Toothpastes often contain fluoride ions to help
protect teeth from attack by bacteria. Some
toothpastes contain Tin(II) fluoride. This compound
has the formula SnF2.
a) Calculate the relative formula mass (Mr) of SnF2.
Show your working.
b) Calculate the percentage by mass of fluorine in
SnF2
c) A tube of toothpaste contains 1.2g of SnF2.
Calculate the mass of fluorine in this toothpaste.
Show your working.
Exam question
Task three
• Read through pages 116-117 and answer
questions in the summary box