1. C4b Reacting Masses
Revision
1. What do we mean by the ‘atomic mass’ of an element?
2. Looking at the periodic table and find the relative atomic masses of a) – g).
a) He = d) Cl = g) O =
b) K = e) Fe = h) K+ =
c) Al = f) Cu = i) Cl- =
3. If you finish quickly:
(I) Attempt h) and i) above
(II) Explain why the mass of chlorine is not a whole number.
Relative formula mass calculations
The relative formula mass of a compound is the total mass of that
compound when the individual masses of the atoms have been
added together.
You must ALWAYS show how you worked it out:
a) H2O = f) Na2SO4 =
b) CO2 = g) KNO3 =
c) NaOH = h) Fe(NO3)3 =
d) CuO = i) Cu(OH)2 =
e) CaCO3 = j) (NH4)2SO4 =

2. How much copper oxide can you get from copper carbonate?
Aim
You are going to find out how much copper oxide you can get by heating copper
carbonate in a crucible.
Method
1 Weigh an empty crucible with its lid. Record the mass of crucible + lid
2 Using a spatula, place about 3 to 4 g of copper(II) carbonate in the crucible. Weigh
the crucible + lid + contents. Record this mass.
3 Set up the apparatus as shown.
SAFETY
Copper carbonate – harmful
irritant
Copper oxide - irritant
HOT !!
4 Heat the crucible gently at first and then more strongly.
5 After heating for about one minute, remove the Bunsen and allow the crucible to
cool for about 20 seconds. Then use the tongs to lift the lid slightly. Observe the
contents. Do not breathe in any of the dust.
6 Stir the contents of the crucible gently with a clean dry spatula so that no dust is
raised and then replace the lid.
7 Repeat steps 4 to 6 until you cannot see any blue-green powder in the crucible.
8 Allow the crucible to cool completely then reweigh the crucible + lid + contents.
Analysis of your results
1 Use your knowledge of the decomposition of transition metal carbonates to write a
word equation for the reaction which occurs when copper(II) carbonate is heated.

3. 2 What will happen to the mass of the crucible as the reaction occurs? Explain your
answer.
3 Calculate the mass of copper(II) carbonate you started with:
mass of copper(II) carbonate = mass recorded in step 2 − mass recorded in step 1
4 Calculate the mass of copper(II) oxide product:
mass of copper(II) oxide = mass recorded in step 8 − mass recorded in step 1
5 Use a spreadsheet to calculate the expected yield of copper oxide and the
percentage yield. Your teacher will show you how to do this.
6 Suggest reasons why your percentage yield was lower than 100%.
Summary
• The greater the amount of starting materials (reactants) used, the ___________
the amount of new substances (products) formed.
• The total mass of reactants at the start of a reaction is ________________ the
total mass of products made (Law of Conservation of Matter)
• Mass is conserved because matter cannot be _______________ or__________
it can only change from one form to another. So, if you put 25 atoms in at the
start of an experiment then you will get the same _____ atoms out, even of they
have rearranged into new substances.
__ atoms of oxygen have reacted with __
e.g. O2 + 2H2 2H2O of hydrogen to make 2 molecules of water
but there are still __ O and __ H
• Percentage yield can be found using the formula:
percentage yield = actual yield x 100
predicted yield
• Where is some of the product lost in reactions?
1.
2.
3.
4.