Year 9 Chemistry, Worksheet 21
5.2 Mass ratios and the molar volume
We can use balanced molecular and chemical equations to predict the masses of reactants and products as well as the other way around.
Example:
How many grams of oxygen (O2) react with 32 grams of methane (CH4) in a complete combustion (see right)?
Given: m(CH4) = 32 g Needed: m(O2) = ?
Calculation: M(CH4)= M(C) + 4*M(H) = 12 g/mol + 4*(1 g/mol) = 16 g/mol M(O2) = 2*M(O) = 2*(16 g/mol) = 32 g/mol
n(CH4) = m(CH4)/M(CH4) = 32 g / (16 g/mol) = 2 mol From chemical equation: n(O2) = 2*n(CH4)
n(O2) = 2*(2 mol) = 4 mol
m(O2) = n(O2)* M(O2) = 4 mol * 32 g/mol = 128 g
Tasks:
1. Chlorine reacts with calcium to produce calcium chloride:
Ca + Cl2 CaCl2
a. Calculate the mass of calcium that reacts with 10 g of chlorine.
b. Calculate the mass of chlorine needed to produce 15 g of calcium chloride.
2. Aluminium reacts with sulfur as follows:
2 Al + 3 S Al2S3
a. Calculate the mass of sulfur that reacts with 20 g of aluminium
b. Calculate the mass of aluminium sulfide that is produced when 15 g of sulfur react with aluminium.
3. The oxygen used up in reactions is a vital in closed spaces with an air supply, e.g. a submarine or a spaceship. The cells (e.g. muscles) in human bodies use up oxygen by reacting with nutrients such as glucose (C6H12O6). The equation for this reaction is
C6H12O6 + 6 O2 6 CO2 + 6 H2O
a. Calculate the mass of glucose that can be burnt in a submarine with an oxygen supply of 150 kg ( = ? g) before it runs out of oxygen.
b. Calculate the mass of carbon dioxide produced before the submarine runs out of oxygen.
Year 9 Chemistry, Worksheet 21
We can use the molar volume to calculate substance amounts and volumes of gaseous reactants and products.
By the way: Since it’s important to know if the involved elements are mono- or diatomic gases (since the molar volume gives us the volume for one mole of molecules), we should introduce a way to memorize them.
The diatomic substances: Have No Fear Of Ice Cold Beer So: H2, N2, F2, O2, I2, Cl2, Br2
Tasks:
1. Chlorine reacts with calcium to produce calcium chloride:
Ca + Cl2 CaCl2
a. Calculate the volume of chlorine gas that reacts with 15 g of magnesium.
b. Calculate the mass of magnesium chloride that is produced using 500 ml of chloride gas.
2. The oxygen used up in reactions is a vital in closed spaces with an air supply, e.g. a submarine or a spaceship. The cells (e.g. muscles) in human bodies use up oxygen by reacting with nutrients such as glucose (C6H12O6). The equation for this reaction is
C6H12O6 + 6 O2 6 CO2 + 6 H2O
a. Calculate the mass of glucose that can be burnt in a submarine with an oxygen supply of 2000 l ( = ? g) before it runs out of oxygen.
b. Calculate the volume of carbon dioxide produced before the submarine runs out of oxygen.
