Stoichiometry Of Reactions

CONTENT

• Calculation of masses of reactants and products
• Calculation of volume of reacting gases

STOICHIOMETRY OF REACTIONS

The calculation of the amounts (generally measured in moles or grams) of reactants and products involved in a chemical reaction is known as stoichiometry of reaction. In other words, the mole ratio in which reactants combine and products are formed gives the stoichiometry of the reactions.

From the stoichiometry of a given balanced chemical equation, the mass or volume of the reactant needed for the reaction or products formed can be calculated.

CALCULATION OF MASSES OF REACTANTS AND PRODUCTS

1. Calculate the mass of solid product obtained when 16.8g of NaHCO3 was heated strongly     until there was no further change.

    Solution:

    The equation for the reaction is:

    2NaHCO_3(s) → Na₂CO_3(s) + H₂O_(g) CO_2(g)

    Molar mass of NaHCO₃ = 23 + 12 + 16×3 = 84gmol^-1

    Molar mass of Na₂CO₃ = 23×2 +12+16×3 = 106gmol^-1

    From the equation:

    2 moles NaHCO₃ produces 1 mole Na₂CO₃

    2x84g NaHCO₃ produces 106g Na₂CO₃

    16.8g NaHCO₃ will produce Xg Na₂CO₃

     Xg Na₂CO₃ = (106g x 16.8g)/2 x 84g       =10.6g

     Mass of solid product obtained = 10.6g

2. Calculate the number of moles of CaCl₂ that can be obtained from 25g of limestone [CaCO₃] in the presence of excess acid.

     Solution:

      The equation for the reaction is:

       CaCO_3(s) + 2HCl → CaCl_2(s) + H₂0_(l) + CO_2(g)

       Number of moles = Reacting mass/Molar mass

       Molar mass of CaCO₃ = 40 + 12 + 16×3 = 100gmol^-1

       Number of moles of CaCO₃ = 25g/100gmol^-1   = 0.25 mole

    From the equation of reaction,

    1 mole CaCO₃ yields 1 mole CaCl₂

    Therefore, 0.25 mole CaCO₃ yielded 0.25 mole CaCl_2.

EVALUATION

1. What does the term ‘Stoichiometry of reaction’ mean?
2. Ethane [C₂H₆] burns completely in oxygen. what amount in moles of CO₂ will be produced when 6.0g of ethane are completely burnt in oxygen

CALCULATION OF VOLUME OF REACTING GASES

 1. In an experiment, 10cm³ of ethene [C₂H₄] was burnt in 50cm³ of oxygen.

A. Which gas was supplied in excess? Calculate the volume of the excess gas remaining at the end of the reaction.

B. Calculate the volume of CO₂ gas produced.

Solution:

The equation for the reaction is:

C₂H_4(g) + 3O_2(g) → 2CO_2(g) + 2H₂O_(g)

From the equation,

A.1 mole of ethene reacts with 3mole of oxygen

     1 volume of ethene reacts with 3 volumes of oxygen

    10cm³of ethene will react with 30cm³ of oxygen

    Since 50cm³ of oxygen was supplied, oxygen was in excess

    Hence volume of the excess gas = initial volume – volume used up = 50-30 = 20cm³

B.1 volume of ethene produces 2 volumes of CO₂

    10 cm³ of ethene will produce 20cm³ of CO₂

    Therefore, 20cm³ of CO₂ was produced

2. 20cm³ of CO was mixed and sparked with 200cm³ of air containing 21% of O₂. If all the     volumes are measured at s.t.p, calculate the total volume of the resulting gases.

     Solution:

     In 200cm³ of air,

    Volume of O₂ = 21   x 200cm³ = 42cm³

     100

Volume of N₂ and rare gases = 200-42 = 158cm³

The equation for the reaction is:

             2CO_(g) + O_2(g) → 2CO_2(g)

Volume ratio            2       :   1     :     2

Before sparking     20cm³   42cm³

Reacting volume     20cm³   10cm³

After sparking                    32cm³   20cm³

Volume of resulting gases = 32 + 20 + 158 = 210cm³

GENERAL EVALUATION/REVISION

1. Find the volume of oxygen produced by 1 mole of KClO3 at s.t.p in the following reaction:      2KClO_3(s)  → 2KCl_(s) + 30_2(g)

2. Balance the following equations:   (a) Cu₂S_(s) +O_2(g) → Cu₂O_(s) + SO_2(g)

                                                            (b) C_(s) + H₂O_(g) → CO_(g) + H_2(g)                   

3. Write the symbols of the following elements: mercury, silver, gold, lead, tin, antimony.

4. Define the term valency

READING ASSIGNMENTNew School Chemistry for Senior Secondary School by

 O. Y. Ababio, Pg 156-164

WEEKEND ASSIGNMENT

1. Amount of a substance is expressed in A. mole B. grams C. kilograms D. mass
2. Determine the mass of CO₂produced by burning 104g of ethyne [C2H2]A. 256g B.352g C. 416g D. 512g
3. The mole ratio in which reactants combine and products are formed is known as A. rate of reaction

B. stoichiometry of reaction C. equation of reaction D. chemical reaction

• The unit for relative molecular mass is A. mole B. gmol^-1C. grams D. mass
• What mass of Pb(NO₃) would be required to 9g of PbCl₂ on the addition of excess NaCl solution? [Pb=207, Na=23, O=16, N=14] A. 10.7g B. 1.2g C. 6.4g D. 5.2g

THEORY

1. Calculate the number of molecules of CO₂produced when 10g of CaCO₃ is treated with 100cm³ of 0.20moldm^-3 HCl.
2. Calculate the volume of nitrogen that will be produced at s.t.p from the decomposition of 9.60g ammonium dioxonitrate(iii), NH₄NO₂.