Embedded Files
Objectives
Objectives
Use the molar gas volume, taken as 24dm3 at room temperature and pressure (R.T.P.)
Calculate stoichiometric volumes of gases
A reminder to please only use your IGCSE periodic table for stoichiometry.
Mass and Molar Mass calculations
Mass and Molar Mass calculations
You must memorize this triangle. It is quicker with the symbols if you can.
Mass, m (g)
Molar Mass, M (g/mol)
Number of moles, n (mol)
Definitions to remember
Definitions to remember
1 dm3 = 1 Litre = 1000 cm3
To change from cm3 to dm3, divide cm3 by 1000.
One mole of any gas will occupy a volume of 24 dm3 at room temperature and pressure (r.t.p.). Thus, we can say that the molar volume of any gas is 24 dm3/mol at r.t.p.
Calculating gas volumes
Calculating gas volumes
This video series is better for gas volumes (the Tyler DeWitt videos we've used previously in stoichiometry use a different molar volume which might be confusing)
Practice with moles
Practice with moles
What volume does 4.5mol of O2 occupy, assuming it is at room temperature and pressure (r.t.p.)?
Answer
Answer
V = 4.5mol x 24dm3/mol = 108dm3
ANSWER: The volume of the O2 will be 108dm3
2. How many moles are in 150cm3 of CO2, assuming it is at r.t.p.?
Answer
Answer
n = V = 0.150dm3 = 0.00625 mol = 6.25 x 10-3 mol
Vm 24dm3/mol
ANSWER: There are 6.25 x 10-3 moles of CO2.
Stoichiometric gas volumes
Stoichiometric gas volumes
Notice that the video examples use both mass and gas volumes. You should practice these too.
The chemical equation will always be a ratio of moles.
Worked example 1
Worked example 1
Solid sulfur is burned in the presence of oxygen to produce sulfur dioxide gas. If 8g of sulfur is burnt, what volume of SO2 is produced?
Write word and symbol equation
Balance equation and write what you know
Convert mass of sulfur to moles of sulfur
Find moles of SO2 produced using moles of sulfur and molar ratio from equation.
Convert moles of sulfur dioxide to volume of sulfur dioxide using the molar volume of gas equation
Answer
Answer
Write word and symbol equation
sulfur + oxygen → sulfur dioxide
S(s) + O2(g) → SO2(g)
Balance equation and write what you know
S(s) + O2(g) → SO2(g) (already balanced)
m=8g V=?
Vm = 24dm3/mol
Convert mass of sulfur to moles of sulfur
n(S) = m = __8g _ = 0.25 mol of sulfur
M 32g/mol
Find moles of SO2 produced using moles of sulfur and molar ratio from equation.
S(s) + O2(g) → SO2(g)
↓x0.25 ↓x0.25 ↓x0.25
0.25 S(s) 0.25 O2(g) 0.25 SO2
Convert moles of sulfur dioxide to volume of sulfur dioxide using the molar volume of gas equation
n(SO2) = V → V = n(SO2) x Vm = 0.25 mol x 24dm3/mol = 6dm3
Vm
ANSWER: The volume of sulfur dioxide produced is 6 dm3.
Worked example 2
Worked example 2
Hydrogen gas (H2) and chlorine gas (Cl2) react to form hydrogen chloride gas (HCl). What volume of hydrogen chloride would be made if 30dm3 of hydrogen was used?
Write word and symbol equation
Balance equation and write what you know
Convert volume of hydrogen to moles of hydrogen
Find moles of HCl produced using moles of H2 and molar ratio from equation.
Convert moles of HCl to volume of HCl
Answer
Answer
Write word and symbol equation
hydrogen + chlorine → hydrogen chloride
H2(g) + Cl2(g) → HCl(g)
Balance equation and write what you know
H2(g) + Cl2(g) → 2HCl(g)
V=30dm3 V=?
Vm = 24dm3/mol Vm = 24dm3/mol
Convert volume of hydrogen to moles of hydrogen
n(H2) = V = 30dm3 = 1.25 mol of H2
Vm 24dm3
Find moles of HCl produced using moles of H2 and molar ratio from equation.
H2(g) + Cl2(g) → 2HCl(g)
↓x1.25 ↓x1.25 ↓x1.25
1.25 H2(g) 1.25 Cl2(g) 2.5 HCl(g)
Convert moles of HCl to volume of HCl
n(HCl) = V → V = n(HCl) x Vm = 2.5 mol x 24dm3/mol = 60dm3
Vm
ANSWER: The volume of HCl produced is 60 dm3
Practice questions
Practice questions
Solid magnesium carbonate decomposes (breaks down) when heated to form solid magnesium oxide and carbon dioxide gas.
Write a balanced chemical equation for this reaction
What volume of CO2 is made if 2.7g of magnesium carbonate is used.
Ethane gas (C2H6) combusts with oxygen (O2) to form carbon dioxide and water vapour. The equation is:
2C2H6(g) + 7O2(g) → 4CO2(g) + 6H2O(g)
Calculate the volume of carbon dioxide formed if 4.50mol of C2H6 is used.
Calculate the volume of water vapour formed if 7.38dm3 of C2H6 is used.
Answers
Answers
1a) MgCO3(s) → MgO(s) + CO2(s)
1b) 0.77dm3 of CO2
2a) 216dm3 of CO2
2b) 22.1dm3 of H2O
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