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15.1 Stoichiometry and composition  (Page 4/5)

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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))

  1. C 3 H 8 ( g ) + 5 O 2 ( g ) → 3 C O 2 ( g ) + 4 H 2 O ( g )

  2. From the balanced equation, the ratio of oxygen to propane in the reactants is 5:1.

  3. 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?

  1. Fe ( s ) + S ( s ) → FeS ( s )

  2. n = m M = 5 , 6 55 , 85 = 0 , 1 mol

  3. 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.

  4. m = n × M = 0 , 1 × 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 ) → ( 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?

  1. n ( H 2 SO 4 ) = m M = 2 000 g 98 , 078 g · mols - 1 = 20 , 39 mols

  2. 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 × M = 20 , 41 mol × 132 g · 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

  1. 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 ) → 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?
  2. Sodium azide is a commonly used compound in airbags. When triggered, it has the following reaction: 2 NaN 3 ( s ) → 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?
  3. 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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