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3.
Law of Conservation of Mass
Calculations
Help for problem solving
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questions on the law of conservation of mass in chemical reactions using
the balanced equation. The Law of Conservation of
Mass is defined and explained using examples of reacting mass calculations using the law
are fully explained with worked out examples using the balanced symbol
equation. The method involves reacting masses deduced from the balanced
symbol equation.
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3.
Law of Conservation of mass
calculations
 What is the Law of Conservation of Mass?
 When elements and compounds react to form
new products, mass cannot be lost or gained.
 "The Law of Conservation of Mass"
definition states that mass cannot be created
or destroyed, but changed into different forms.
 So, in a chemical change, the total mass of reactants must equal the total mass of products.
 By using this law, together with atomic and formula masses, you can calculate the quantities of reactants and products involved in a
reaction and the simplest formula of a compound
NOTE: (1) the symbol equation must be
correctly balanced to get the right answer! (2) There are good reasons why, when
doing a real chemical preparationreaction to make a substance you will not get
100% of what you theoretically calculate.
See discussion in section 14.2
 Law of conservation of mass
calculation Example 3.1
 Magnesium + Oxygen ==> Magnesium oxide
 2Mg + O_{2}
==> 2MgO (atomic masses required: Mg=24 and O=16)
 think of the ==> as an = sign, so the mass changes in the reaction are:
 (2 x 24) + (2 x 16) = 2 x (24 + 16)
 48 + 32 = 2 x 40 and so 80 mass units of
reactants = or produces 80 mass units of products (you can work with any mass
units such as g, kg or tonne (1 tonne = 1000 kg)
 Law of conservation of mass
calculation Example 3.2
 iron +
sulphur ==> iron sulphide (see the diagram at the top of
the page!)
 Fe + S ==> FeS (atomic
masses: Fe = 56, S = 32)
 If 59g of iron is heated with
32g of sulphur to form iron sulphide, how much iron is left unreacted?
(assuming all the sulphur reacted)
 From the atomic masses, 56g of
Fe combines with 32g of S to give 88g FeS.
 This means 59  56 = 3g Fe
unreacted.
 Law of conservation of mass
calculation Example 3.3
 When limestone (calcium carbonate) is strongly heated, it undergoes
thermal decomposition to form lime (calcium oxide) and carbon dioxide gas.
 CaCO_{3}
==>
CaO + CO_{2} (relative atomic masses: Ca = 40, C = 12 and O = 16)
 Calculate the mass of calcium oxide and the mass of carbon dioxide formed by decomposing 50 tonnes of calcium carbonate.
 (40 + 12 + 3x16)
==> (40 + 16) + (12 + 2x16)
 100 ==>
56 + 44
 scaling down by a factor of
two
 gives
 50 ==>
28 + 22
 so decomposing 50 tonnes of limestone produces
28 tonnes of lime and 22 tonnes of carbon dioxide gas.
 Example 3.4:
 For more complicated examples and
more practice of calculations based on the Law of Conservation of
Mass
 SEE
Reacting mass ratio calculations of reactants and products from
equations (NOT using moles)
Selfassessment Quizzes
[com] type in answer
for
F and H or
multiple choice
for
F and H
OTHER CALCULATION PAGES

What is relative atomic mass?,
relative isotopic mass and calculating relative atomic mass

Calculating relative
formula/molecular mass of a compound or element molecule

Law of Conservation of Mass and simple reacting mass calculations
(this page)

Composition by percentage mass of elements
in a compound

Empirical formula and formula mass of a compound from reacting masses
(easy start, not using moles)

Reacting mass ratio calculations of reactants and products
from equations
(NOT using
moles) and brief mention of actual percent % yield and theoretical yield,
atom economy
and formula mass determination

Introducing moles: The connection between moles, mass and formula mass  the basis of reacting mole ratio calculations
(relating reacting masses and formula
mass)

Using
moles to calculate empirical formula and deduce molecular formula of a compound/molecule
(starting with reacting masses or % composition)

Moles and the molar volume of a gas, Avogadro's Law

Reacting gas volume
ratios, Avogadro's Law
and GayLussac's Law (ratio of gaseous
reactantsproducts)

Molarity, volumes and solution
concentrations (and diagrams of apparatus)

How to do acidalkali
titration calculations, diagrams of apparatus, details of procedures

Electrolysis products calculations (negative cathode and positive anode products)

Other calculations
e.g. % purity, % percentage & theoretical yield, dilution of solutions
(and diagrams of apparatus), water of crystallisation, quantity of reactants
required, atom economy

Energy transfers in physical/chemical changes,
exothermic/endothermic reactions

Gas calculations involving PVT relationships,
Boyle's and Charles Laws

Radioactivity & halflife calculations including
dating materials
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