Answer:

Calculating the mass of products

Example

2.43 g of magnesium reacts completely with excess hydrochloric acid to form magnesium chloride and hydrogen:

Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)

Calculate the maximum mass of hydrogen that can be produced. (Relative masses: Mg = 24.3, H2 = 2.0)

number of moles of Mg = 2.23

= 0.100 mol

Looking at the balanced chemical equation, 1 mol of Mg forms 1 mol of H2, so 0.100 mol of Mg forms 0.100 mol of H2

mass of H2 = Mr × number of moles of H2

= 2.0 × 0.100

= 0.20 g (to 2 significant figures)

Example

1.0 g of calcium carbonate decomposes to form calcium oxide and carbon dioxide:

CaCO3(g) CaO(s) + CO2(g)

Calculate the maximum mass of carbon dioxide that can be produced. (Relative formula masses: CaCO3 = 100.1, CO2 = 44.0)

number of moles of CaCO3 =

=

= 0.010 mol (to 2 significant figures)

Looking at the balanced chemical equation, 1 mol of CaCO3 forms 1 mol of CO2, so 0.010 mol of CaCO3 forms 0.010 mol of CO2

mass of CO2 = Mr x number of moles of CO2

= 44.0 × 0.010

= 0.44 g

Answer:

12.2 g of magnesium metal (Mg(s)) reacts completely with oxygen gas (O2(g)) to produce magnesium oxide (MgO(s)).

Calculate the mass of oxygen consumed during the reaction and the mass of magnesium oxide produced.

1) Write the balanced chemical equation for the chemical reaction:

2Mg(s) + O2(g) → 2MgO(s)

(2) Determine the mole ratio (stoichiometric ratio) from the equation, Mg : O2 : MgO

moles(Mg) : moles(O2) : moles(MgO) is 2:1:2

(3) Use the mole ratios to calculate the mass of O2 consumed and MgO produced as shown below:

mass O2 = moles(O2) × molar mass(O2)

(a) Calculate moles(Mg) = mass(Mg) ÷ molar mass(Mg)

moles(Mg) = 12.2 ÷ 24.31 = 0.50 mol

(b) Use the balanced chemical equation to determine the mole ratio O2:Mg

moles(O2) : moles(Mg) is 1:2

(c) Use the mole ratio to calculate moles O2

moles(O2) = (1 ÷ 2) × moles(Mg)

substitute in the value for moles of Mg

moles(O2) = ½ × 0.50 = 0.25 mol

(d) Calculate mass O2

mass(O2) = moles(O2) × molar mass(O2)

mass(O2) = 0.25 × (2 × 16.00) = 0.25 × 32.00

mass(O2) = 8.03 g

mass MgO = moles(MgO) × molar mass(MgO)

(a) Calculate moles Mg

moles(Mg) = mass(Mg) ÷ molar mass(Mg)

moles(Mg) = 12.2 ÷ 24.31 = 0.50 mol

(b) Use the balanced equation to determine the mole ratio MgO:Mg

moles(MgO) : moles(Mg) is 2:2 which is the same as 1:1

(c) Use the mole ratio to calculate moles MgO

moles(MgO) = 1 × moles(Mg)

substitute in the value for moles of Mg

moles(MgO) = 1 × 0.50 = 0.50 mol

(d) Calculate mass MgO

mass(MgO) = moles(MgO) × molar mass(MgO)

mass(MgO) = (1 × 0.50) × (24.31 + 16.00) = (1 × 0.50) × 40.31

mass(MgO) = 20.16 g

Explanation:

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