Calculating masses used in reactions
How much fuel should the space
shuttle take on a trip?
Aluminum + oxygen
Aluminum oxide
Why does NASA have to provide exactly the
right amount of alumiuium?
How do they do this?
Calculating masses used in reactions
In chemical reactions, a certain amount of one reactant combines with a
certain amount of another, to make a certain amount of product(s).
You can use RAM, RFM and the balanced symbol equation to predict
masses of products formed, or masses of reactants used up. If you are
to be successful, you must have the correct balanced equation.
Otherwise, promise, you don’t stand a chance. You have been warned.
[Student print-out of questions and answers to all examples below]
Example 1: What mass of magnesium oxide is produced when 60g of
magnesium is burned in air?
1. Write the equation:
2Mg + O2
2. Highlight the two
chemicals involved
3. Write down the mole ratio (from equation)
4. Work out no. of moles
of the known substance
2MgO
5. Apply mole ratio to find number
of moles of unknown substance
6. Hence calculate
mass of unknown substance
Write down your answer. It is telling you the following: if I burn 60g of
magnesium in air, I can expect to obtain ____g of magnesium oxide.
Example 2: What mass of calcium is needed to give 30g of calcium oxide
when burnt in air?
1. Write the equation and highlight:
2. Write down the mole ratio
3. Work out no. of moles
of the known substance
2Ca + O2
2CaO
4. Apply mole ratio to find no.
of moles of unknown substance
5. Hence calculate mass of unknown
substance
So, if I want to form 30g of calcium oxide, I need to burn ____g of
calcium in air.
Other examples:
3. Zinc oxide is heated with carbon, producing zinc and carbon
monoxide. How much zinc oxide do you need to make 130g of zinc?
A.
B.
C.
D.
E.
Equation, and highlighted elements:
Mole ratio:
No. of moles of zinc wanted:
No. of moles of zinc oxide needed:
Actual mass of zinc oxide needed:
ZnO + C
Zn + CO
Answer: To make 130g of zinc, ____g of zinc oxide must be reduced by carbon.
4. Silver nitrate reacts with magnesium chloride forming silver
chloride and magnesium nitrate. If you have 0.95g of magnesium
chloride at the start, what is the maximum amount of silver chloride
that could be made?
The equation is:
MgCl2
(aq)
+ __AgNO3
(aq)
__AgCl
(s)
+ Mg(NO3)2
(aq)
A. Copy out the equation and balance it
B. Highlight the two chemicals of interest.
C. Write down the mole ratio according to the equation
D. What does the mole ratio actually mean?
E. How much many moles of magnesium
chloride are there at the start?
F. How many moles of silver chloride can
you expect to make from this?
G. So, what mass of silver chloride can you
make from 0.95g of magnesium chloride?
Answer: from 0.95g of magnesium chloride, I can make _____g of silver
chloride.
5. The balanced symbol equation for the reaction between ammonia
and sulphuric acid is shown below. How much sulphuric acid is
needed to exactly neutralise 340g of ammonia? Show your working
out clearly.
Equation: 2NH3 + H2SO4
(NH4)2SO4
Now work through the Reacting Amount Calculations sheet. See
4.1 Reacting Amounts MORE PRACTICE.PDF For hand-written
answers
1. What mass of magnesium oxide is produced when 60g of
magnesium is burned in air?
2. What mass of calcium is needed to give 30g of calcium oxide
when burnt in air?
3. Zinc oxide is heated with carbon, producing zinc and carbon
monoxide. How much zinc oxide do you need to make 130g of
zinc?
4. Silver nitrate reacts with magnesium chloride forming silver
chloride and magnesium nitrate. If you have 0.95g of magnesium
chloride at the start, what is the maximum amount of silver
chloride that could be made?
5. The balanced symbol equation for the reaction between ammonia
and sulphuric acid is shown below. How much sulphuric acid is
needed to exactly neutralise 340g of ammonia? Show your
working out clearly. Ammonia + sulphuric acid
ammonium sulphate
Now try Exercise 8 Calculation of products / reactants based on equations from pages
71 – 73 of your Student Handbook. Do at least Q1,2,5-8,12,13,15,16. You can try
some of the other questions as well but you may need to look up the equations for some.
Q3 p71 Ans below
1. What mass of magnesium oxide is produced when 60g of
magnesium is burned in air?
2. What mass of calcium is needed to give 30g of calcium oxide
when burnt in air?
3. Zinc oxide is heated with carbon, producing zinc and carbon
monoxide. How much zinc oxide do you need to make 130g of
zinc?
4. Silver nitrate reacts with magnesium chloride forming silver
chloride and magnesium nitrate. If you have 0.95g of magnesium
chloride at the start, what is the maximum amount of silver
chloride that could be made?
5. The balanced symbol equation for the reaction between ammonia
and sulphuric acid is shown below. How much sulphuric acid is
needed to exactly neutralise 340g of ammonia? Show your
working out clearly. Ammonia + sulphuric acid
ammonium sulphate
Now try Exercise 8 Calculation of products / reactants based on equations from pages
71 – 73 of your Student Handbook. Do at least Q1,2,5-8,12,13,15,16. You can try
some of the other questions as well but you may need to look up the equations for some.
Q3 p71 Ans below
Answers to examples 1 – 5 from my sheet and to CGP sheet below:
And one more example below: