15.1 Stoichiometry and composition (Page 4/5)

Siyavula textbooks: grade 10 Page 4 / 5

What volume of oxygen at S.T.P. is needed for the complete combustion of $2 {dm}^{3}$ of propane ( $C_{3} H_{8}$ )? (Hint: $CO_{2}$ and $H_{2} O$ are the products in this reaction (and in all combustion reactions))

Write a balanced equation for the reaction.
$C_{3} H_{8} (g) + 5 O_{2} (g) \to 3 C O_{2} (g) + 4 H_{2} O (g)$
Determine the ratio of oxygen to propane that is needed for the reaction.
From the balanced equation, the ratio of oxygen to propane in the reactants is 5:1.
Determine the volume of oxygen needed for the reaction.
1 volume of propane needs 5 volumes of oxygen, therefore $2 {dm}^{3}$ of propane will need $10 {dm}^{3}$ of oxygen for the reaction to proceed to completion.

What mass of iron (II) sulphide is formed when $5,6 g$ of iron is completely reacted with sulphur?

Write a balanced chemical equation for the reaction.
$Fe (s) + S (s) \to FeS (s)$
Calculate the number of moles of iron that react.
$n = \frac{m}{M} = \frac{5, 6}{55, 85} = 0, 1 mol$
Determine the number of moles of $FeS$ produced.
From the equation $1 mole$ of $Fe$ gives $1 mole$ of $FeS$ . Therefore, $0,1 moles$ of iron in the reactants will give $0,1 moles$ of iron sulphide in the product.
Calculate the mass of iron sulphide formed
$m = n \times M = 0, 1 \times 87, 911 = 8, 79 g$

The mass of iron (II) sulphide that is produced during this reaction is $8,79 g$ .

When we are given a known mass of a reactant and are asked to work out how much product is formed, we are working out the theoretical yield of the reaction. In the laboratory chemists never get this amount of product. In each step of a reaction a small amount of product and reactants is 'lost' either because a reactant did not completely react or some of the product was left behind in the original container. Think about this. When you make your lunch or supper, you might be a bit hungry, so you eat some of the food that you are preparing. So instead of getting the full amount of food out (theoretical yield) that you started preparing, you lose some along the way.

Sulphuric acid ( $H_{2} SO_{4}$ ) reacts with ammonia ( $NH_{3}$ ) to produce the fertiliser ammonium sulphate (( $NH_{4}$ ) $_{2} SO_{4}$ ) according to the following equation:

$H_{2} {SO}_{4} (aq) + 2 {NH}_{3} (g) \to {({NH}_{4})}_{2} {SO}_{4} (aq)$

What is the maximum mass of ammonium sulphate that can be obtained from $2,0 kg$ of sulphuric acid?

Convert the mass of sulphuric acid into moles
$n (H_{2} {SO}_{4}) = \frac{m}{M} = \frac{2 000 g}{98, 078 g \cdot {mols}^{- 1}} = 20, 39 mols$
Calculate the maximum amount of ammonium sulphate that can be produced
From the balanced equation, the mole ratio of $H_{2} SO_{4}$ in the reactants to $(NH_{4})_{2} SO_{4}$ in the product is 1:1. Therefore, $20,39 mols$ of $H_{2} SO_{4}$ of $(NH_{4})_{2} SO_{4}$ .

The maximum mass of ammonium sulphate that can be produced is calculated as follows:
$m = n \times M = 20, 41 mol \times 132 g \cdot {mol}^{- 1} = 2694 g$

The maximum amount of ammonium sulphate that can be produced is $2,694 kg$ .

Khan academy video on stoichiometry - 1

Stoichiometry

Diborane, $B_{2} H_{6}$ , was once considered for use as a rocket fuel. The combustion reaction for diborane is: $B_{2} H_{6} (g) + 3 O_{2} (g) \to 2 H B O_{2} (g) + 2 H_{2} O (l)$ If we react $2,37 grams$ of diborane, how many grams of water would we expect to produce?
Sodium azide is a commonly used compound in airbags. When triggered, it has the following reaction: $2 {NaN}_{3} (s) \to 2 Na (s) + 3 N_{2} (g)$ If $23,4 grams$ of sodium azide is used, how many moles of nitrogen gas would we expect to produce?
Photosynthesis is a chemical reaction that is vital to the existence of life on Earth. During photosynthesis, plants and bacteria convert carbon dioxide gas, liquid water, and light into glucose ( C 6 H 12 O 6 ) and oxygen gas.
1. Write down the equation for the photosynthesis reaction.
2. Balance the equation.
3. If $3 moles$ of carbon dioxide are used up in the photosynthesis reaction, what mass of glucose will be produced?

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Source: OpenStax, Siyavula textbooks: grade 10 physical science [caps]. OpenStax CNX. Sep 30, 2011 Download for free at http://cnx.org/content/col11305/1.7

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