Chapter 25 - Chemical Formulas and Equations
Section A – Types of Chemical Formula
Structural Formula
Molecular Formula
Empirical Formula
Section B – Calculating Empirical Formulas
Method 1 – If given the % composition by mass of each element present in the compound
1. Divide the percenntage mass of each element by its _______________________
2. Find the simplest ___________________________________ between the
elements present.
Example 1
A compound contains 40% sulfur and 60 % oxygen by mass. What is its empirical formula?
Element
%
moles
sulfur
40
40/
oxygen
60
60/
Ratio
= 1.25
Ans: Therefore the empirical Formula is
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Chapter 25 - Chemical Formulas and Equations
Method 2 – Given the masses of reactants and products
Key idea:
Total mass of reactants =
1. Use the masses of reactants and products given to figure out the mass of
remaining reactants.
2. Divide the percenntage mass of each element by its _______________________
3. Find the simplest ___________________________________ between the
elements present.
Example 2
When 3.175g of copper reacts with chlorine gas 6.725g of copper chloride is formed. Find the
empirical formula of the copper chloride
Answer:
Copper + Chlorine gas
3.175g
Element
mass
Copper
3.175g
Chlorine
3.55g
Copper Chloride
?
6.725g
moles
Ratio
= 0.05
Therefore the empirical formula is
Section C – Molecular Formulas
The molecular formula is a multiple of the empirical formula:
Molecular Formula = x ( Empirical Formula)
x is a ______________ number
*****To find the molecular formula we must know the ___________________ and the
relative molecular mass of the substance.
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Chapter 25 - Chemical Formulas and Equations
Example 1
Urea is used as a fertiliser and an animal feed. It has a relative molecular mass of 60 and is composed
of 46.66% N, 26.66%O, 20%C and 6.66% H. Determine the molecular formula of urea
Element
%
moles
ratio
nitrogen
46.66
3.33
2
oxygen
26.66
1.66
1
Carbon
20
1.66
1
Hydrogen
6.66
6.66
4


Mass according to Empirical Formula is
Relative molecular mass of urea =

Empirical formula = x ( Empirical Formula)
X=
Molecular formula is
Section D – Percentage composition by mass
The following formula can be used for this:
% composition of mass =
______________________________________ x 100
Example 1
What is the percentage by mass of Fe in Fe2O3?
Mass of Fe in compound
x 100
Relative molecular mass of the Fe2O3
___________________________
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Chapter 25 - Chemical Formulas and Equations
Section E – Balancing chemical equations
For an equation to be balanced:
total number of atoms of each element of reactant must equal the total number of atoms of
that element in the product
Rules:
1. Chemical formulas can’t be ________________
2. Chemical formulas can be ________________by a suitable number
Example 1
Balance the following equation
Step 1: See what is present:
Fe + O2
Fe3O4
Reactants
Products
Oxygen atoms:
Iron atoms:
Step 2 Make changes to balance the equation:
Section F – Balancing redox equations
Assigning oxidation numbers can help to balance redox equations.
In a redox reaction the number of electrons ____________ from one substance during a
reaction must ___________ the amount of electrons __________ by another substance.
How to balance a redox equation
1.
2.
3.
4.
Assign oxidation numbers to all the species involved in the reaction.
Identify what is oxidised (number increases), and how much by.
Identify what is reduced (number decreases), and how much by.
Identify the ratio needed between what is oxidised and what is reduced so that the
total increase in oxidation number is the same as the total decrease in oxidation
number.
5. Balance the rest of the equation maintaining the ratio decided on in 4.
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Chapter 25 - Chemical Formulas and Equations
Example 1
MnO4- + Cl- + H+
Mn+2 +
Cl2 + H2O
Section G – Calculations based on Chemical equations
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Chapter 25 - Chemical Formulas and Equations
Example 1
The fermentation of glucose results in the formation of ethanol and carbon dioxide according to the
equation:
C6H1206
2C2H5OH + 2CO2
If 126g of glucose are consumed
(i) How many moles of glucose does this represent?
(ii) How many moles of ethanol are produced?
(iii) What volume of carbon dioxide, measured at s.t.p is produced?
(iv) How many molecules of carbon dioxide does this volume contain?
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Chapter 25 - Chemical Formulas and Equations
Section H – Reactions involving one reagent in excess
Reagent in excess
Limiting reagent
example 1: Ammonia gas can be prepared from ammonium chloride and sodium hydroxide:
NH4Cl + NaOH
NH3 + NaCl + H20
If 20.7g of ammonium chloride and 10g of sodium hydroxide are used answer the following
questions:
Answer
(i) which reagent is in excess?
(ii) which reagent is the limiting reagent?
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Chapter 25 - Chemical Formulas and Equations
(iii) Calculate the mass of sodium chloride which would be generated in the reaction
(iv) the volume of ammonia formed at s.t.p made in the reaction
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Chapter 25 - Chemical Formulas and Equations
Section I – Percentage Yield
Percentage Yield = ______________________
Example 1 – In an experiment to prepare ethane 10.2g of ethanol was heated with
aluminium oxide and 1.7g of ethene was formed. Calculate the percentage yield of ethene
C2H5OH
C2H4 + H20
Answer
(i)
Calculate the theoretical yield:
(ii)
Calculate the actual yield
(iii)
Finding the percentage yield:
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Chapter 25 - Chemical Formulas and Equations
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Definition of empirical formula
Calculations of empirical formulas, given
the percentage composition by mass.
Calculations of empirical formulas, given
the mass of reactants and products
Definition of molecular formula
Calculation of molecular formulas
Calculations of the Percentage composition
by mass given the empirical formula
Definition of Structural formulas and simple
examples.
How to balance a simple chemical equation
How to balance redox equations using
oxidation numbers
Solve problems involving chemical
equations when one reactant is in excess
Be able to solve problems involving
chemical equations by doing calculations
using moles, mass, volumes and numbers of
particles.
Calculations involving % yields ( see
organic chemistry section)
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Chapter 25 - Chemical Formulas and Equations
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