1. b) Calculation of the mass or volume of products
Learning intention
 Learn how the theoretical mass or volume of product can be calculated from the balanced reaction equation.

2. 3.12 Calculations from Equations
Learn how chemists calculate how much reactant is required and how much product this will produce using a balanced chemical equation.
Learning Outcomes/Success Criteria Unit 3 CfE Higher Chemistry R A G
I can balance chemical equations and use these to calculate the mass of a reactant or product.
I can use balanced chemical equations to calculate volumes and concentrations
I can carry out calculations involving mass, volumes and concentration
I can carry out calculations involving excess and limiting reagent
I can carry out calculations involving molar gas volumes
I can carry out calculations involving excess volume of gases using a balanced equation

3. From previous studies You should be able to Write Formulae
Calculate percentage composition
Calculate empirical formulae
Calculate the number of moles in a given mass
Calculate the number of moles of solute dissolved in a solution

4. Calculations from Balanced Equations

5. Triangles – Can you draw them?

6. Examples
158.75g
1. Calculate the mass of 2.5mol of copper.

7. 2. 0.5mol l-1
2. Calculate the concentration of a solution containing 2.5mol of substance dissolved in 5l of solution.

8. l
3. 2g of sodium hydroxide, NaOH, are dissolved in water to make 0.4mol l-1 solution. What volume is the solution?

9. Calculations from Equations
A balanced equation is taken to give the relative number of moles of each reactant and product. Since the mass of one mole of any substance is expressed in grams, the masses involved can then be calculated as shown.
Worked Example
Calculate the mass of water produced on burning 1g of methane
CH O2 CO H2O
1 mole moles
It is not necessary to calculate the masses of carbon dioxide and oxygen – these substances are not included in the question.

10. Calculations from Equations

11. g
What mass of copper would be obtained by heating 3.975g of copper (II) oxide with an excess of carbon of carbon?
2CuO + C  2Cu + CO2
Step 1 – Check if the equation is balanced
Step 2 – Write down the molar ratio
Step 3 – Identify what information that you have from the question
Step 4 – Use the appropriate triangle to find the number of moles that you have for the given quantity. Then compare this to the molar ratio for the quantity that you have been asked about.
Step 5 – Now you have the number of moles for the quantity that you are asked about, use the triangle again to find the mass.

12. Calculations based on Equations
What mass of carbon dioxide is produced by the decomposition of 10g calcium carbonate
CaCO3  CaO + CO2
2.What mass of water vapour is produced on burning 7g of ethene (C2H4)
C2H4 + 3O2  2CO2 + 2H2O

13. Reacting Masses Accurately weigh a crucible
Add approx 1.2g of Mg ribbon and reweigh
Place the crucible and lid in a silica triangle.
Heat gently at first then more strongly. Lift the lid with tongs from time to time to admit more oxygen, but not enough to let out the magnesium oxide.

14. Reacting masses
When the reaction is complete, the magnesium will not glow more brightly when the lid is raised
Allow the crucible to cool
Reweigh the crucible

15. Calculations 2Mg + O2 → 2MgO
Using the balanced equation calculate the mass of MgO you would expect to be formed?
2Mg + O2 → 2MgO

16. Moles and Solutions
The molar ratio of a chemical reaction is also useful for reactions involving solutions.
Calculating the mass or number of moles of a reactant or product can also be achieved by using the following formula triangle.

17. What mass of sodium sulfate is formed when 250 cm3 of 0
What mass of sodium sulfate is formed when 250 cm3 of 0.1 mol l-1 sodium hydroxide is neutralised by an excess of sulfuric acid.
The question tells you that there is an excess of acid, so the sodium hydroxide will control how much product is obtained. 2 mol : 1mol 0.025mol : mol
Sodium hydroxide

18. Reacting masses
What mass of sodium carbonate will react completely with 100cm3 of nitric acid concentration 1 mol l-1?
5.3g

19. Reacting masses
What mass of zinc (II) sulphate will be produced on adding 6.0g zinc to excess sulphuric acid?
14.8g

20. Excess Calculations

21. Excess Calculations

22. Excess Calculations

23. The particles are tightly packed together
The particles are tightly packed together. Volume depends on the size of the particles.
solid
liquid
gas

24. The particles are tightly packed together
The particles are tightly packed together. Volume depends on the size of the particles.
solid
The particles are tightly packed together. Volume depends on the size of the particles.
liquid
gas

25. The particles are tightly packed together
The particles are tightly packed together. Volume depends on the size of the particles.
solid
The particles are tightly packed together. Volume depends on the size of the particles.
liquid
The particles are widely spaced out - they occupy only about 1,000th of the volume.
gas

26. The particles are tightly packed together
The particles are tightly packed together. Volume depends on the size of the particles.
solid
The particles are tightly packed together. Volume depends on the size of the particles.
liquid
The particles are widely spaced out - they occupy only about 1,000th of the volume. So volume does not depend on the size of the particles.
gas

27. Count Lorenzo Romano Amedeo Carlo Avogadro
Equal volumes of gases under equal conditions of temperature and pressure contain the same numbers of molecules.

28. Count Lorenzo Romano Amedeo Carlo Avogadro
Provided that the temperature and the pressure are constant, one mole of any gas will occupy the same volume.
The molar volume of all gases contain Avogadro's number 6.02 x 1023 formula units

29. Example 1
A flask was weighed empty (with the air evacuated) and weighed again when filled with CO2 gas. The exact volume of the flask was found by measuring how much water it could hold.
Results:
Mass of empty flask = g
Mass of flask +CO2 = g
So Mass of CO2 = 0.99g
Volume of flask = 570ml (e.g. volume of gas)

30. Measuring Molar gas volume
CO2 Carbon dioxide
Mass of empty flask (g)
Mass of flask + gas (g)
Mass of gas (g)
Volume of flask i.e. volume of gas (l)
Mass of one mole (g)
Volume of 1 mole
Higher Chemistry Eric Alan and John Harris
The volume of the flask can be measured by filling with water and emptying into a measuring cylinder.
Example calculation
You have found that g of CO2 occupies Litres
g ↔ Litres
___ g ↔ Litres
Therefore 1 mole, ____g of CO2 will occupy Litres

31. Example 1 Calculation of molar volume
0.99g of CO2 occupy  570ml
44g (1mole of CO2) occupy  25333ml
So we have found that the molar volume of carbon dioxide is 25.3 litre per mole (l mol-1)
Experiments of this sort with other gases have shown that the molar volume of all gases is the same as long as the temperature and pressure are the same for each.
At 20°C and 1 Atmosphere pressure the molar volume of any gas is 24 litres per mole (l mol-1)

32. Molar Volume
1 mole of O2 = 32
3 mole of O2 = 3 x 32 = 96g

33. Molar Volume
22.2 litres per mole

34. Reacting Volumes

35. Reacting Volumes

36. Molar Volume relationship
Volume of Gas in Litres
n = number of moles
V(l)
Molar Volume in Litres
mol-1 n
Vmol

37. Examples
1.The volume of 0.22g of propene is 118cm3. Calculate the volume of 2mol of propene.
2.The volume of 1g of hydrogen is 11.6 litres. Calculate the volume of 4mol of hydrogen.
3. A flask, capacity 600cm3, was used to calculate the molar volume of sulphur dioxide.
The following data was obtained:
Mass of evacuated flask = g
Mass of flask + sulphur dioxide = g
Calculate the molar volume of sulphur dioxide.
cm3
92.8 litres
cm3

38. 4. From the data calculate the approximate formula mass of gas X
Mass of plastic bottle empty = g
Mass of plastic bottle + gas X = g
Capacity of plastic bottle = 1 litre
Molar volume of gas X = 23.6 litre mol-1
16.992g

39. In Questions 5-7 take the molar volume of the gases to be 23
In Questions 5-7 take the molar volume of the gases to be 23.0 litres mol-1
5. Calculate the volume of
a) 10g of neon
b) 3.2g of oxygen
11.39l
2.3l

40. 6. Phosphorus forms two hydrides, PH3 and P2H4
0.152g of a hydride of phosphorus has a volume of 100cm3
Calculate the mass of one mole of the hydride and identify it.
7. A weather balloon contains equal volumes of helium and hydrogen. If the total volume of the balloon is 9.2 litres, calculate the mass of each gas which will be present in the balloon.
PH3
0.4g – H
0.8 - He