3 chemical formulae and equations

WAJA F4 Chemistry 2010 Chapter 3 : Chemical Formulae and Equations

CHAPTER 3 : CHEMICAL FORMULAE AND EQUATIONS
A RELATIVE ATOMIC MASS (RAM) AND RELATIVE MOLECULAR MASS (RMM)

Learning Outcomes
You should be able to:
state the meaning of relative atomic mass based on carbon-12 scale,
state the meaning of relative molecular mass based on carbon-12 scale,
state why carbon-12 is used as a standard for determining relative atomic
mass and relative molecular mass,
calculate the relative molecular mass of substances.

Activity 1 (refer text book pg 28 )

Relative atomic mass of an element , Ar
= The average mass of an atom of the element
1/12 x the mass of an atom of carbon-12

Example:
Ar of C=12
Ar of O=16
Ar of Mg=24

1. The Relative atomic mass of an element is ……………………………………………………………...

…………………………………. when compare with 1/12 of the mass of an atom of carbon – 12.

2. Carbon-12 is chosen because it is a ………………………. and can be easily handled.

3. Find the relative atomic masses of these elements.

Element Relative Atomic Mass Element Relative Atomic Mass

Calcium, Ca Argon, Ar
Sodium, Na Silver, Ag
Iron, Fe Caesium, Cs
Copper, Cu Lead, Pb
Carbon, C Chlorine, Cl
Hydrogen, H Flourine, F
Potassium, K Aluminium, Al
Lithium, Li Zinc, Zn
Bromine, Br Helium, He


Relative molecular mass of a substance, Mr
= The Average mass of a molecule of the substance
1/12 x the mass of an atom of carbon-12

1


Calculating Relative molecular mass,Mr
Mr= The sum of Ar of all atoms present in one molecule
2 Hydrogen
Molecular atoms
Example:
formula
Mr of Water, H2O = 2(1) + 16 = 18

Relative atomic mass
Relative atomic mass for Oxygen
for Hydrogen

Mr of Carbon dioxide, CO2 = 12 + 2(16) = 44 All Ar, Mr and
Fr have no unit
For ionic substance , Relative formula mass , Fr
= The sum of Ar of all atoms present in the formula

Example:
Fr of Magnesium oxide, MgO = 24 + 16 = 40
Fr of Sodium chloride, NaCl = 23 + 35.5 = 58.5

1. The relative molecular mass of a molecule is ………………………………………………

………………………………………………………. when compared with 1/12 of the mass

of one atom of ……………………………………………

2. Calculate the relative molecular masses of the substances in the table below.

Substance Molecular formula Relative molecular mass, Mr

Hydrogen gas H2 2(1) = 2
Propane C3H8

Ethanol C2H5OH
Bromine gas Br2
Methane CH4
Glucose C6H12O6

Ammonia NH3
[Relative atomic mass : H,1; C,12; O,16; Br,80 ; N,14 ]

3. Calculate the relative formula masses of the following ionic compounds in the table.

Substance Compound formula Relative formula mass, Fr

Potassium oxide K2O 2(39) + 16 = 94

Aluminium sulphate Al2(SO4)3 2(27)+3[32+4(16)]=342

2


Zinc nitrate Zn(NO3)2

Aluminium nitrate Al(NO3)3

Calcium carbonate CaCO3

Calcium hydroxide Ca(OH)2

Hydrated copper(II) CuSO4.5H2O 64 + 32 + 4(16) + 5[2(1) + 16]=250
sulphate

Hydrated sodium Na2CO3.10H2O
carbonate

Sodium hydrogen NaHSO4
sulphate

Aluminium chloride AlCl3

Copper(II) sulphate CuSO4

Zinc carbonate ZnCO3

Potassium K2CO3
carbonate

[Relative atomic mass: O,16; C,12; H,1; K,39 ; Cu,64 ; Zn, 65; Cl, 35.5 ; Al, 27 S,32 ;
Ca, 40; Na,23; N, 14]

B THE MOLE AND THE NUMBER OF PARTICLES

Learning Outcomes
define a mole as the amount of matter that contains as many particles as the
12
number of atoms in 12 g of C,
state the meaning of Avogadro constant,
relate the number of particles in one mole of a substance with the Avogadro
constant,
solve numerical problems to convert the number of moles to the number of
particles of a given substance and vice versa.

3



1. To describe the amount of atoms, ions or molecules , mole is used.

2. A mole is an amount of substance that contains as many particles as the ………………..

…………………………………………………………….. in exactly 12g of carbon-12.

3. A mole is an amount of substance which contains a constant number of particles

atoms, ions, molecules which is 6.02 x 1023

4. The number 6.02 x 1023 is called …………………………………… (NA)

5. In other words:

1 mol of atomic substance contains ……………………………. atoms

1 mol of molecular substance contains ……………………………. molecules

1 mol of ionic substance contains ……

…………………………….. formula units

6. Relationship between number of moles and number of particles (atom/ion/molecules):

x Avogadro Constant
number of moles number of particles

∻ A vogadro Constant

Number of moles Number of particles

0.5 mol of carbon atoms …………………………………… atoms of carbon

0.2 moles of hydrogen gas ( H2) (i) …………………………..molecules
of hydrogen gas
(ii) …………………………….Atoms of hydrogen

2 mol of carbon dioxide molecules ………………x 10 23 molecules of carbon dioxide gas
contains :

………………. atoms of C and

…………………. atoms of O

4


0.007 mol of calcium ions ……………………… calcium ions

…………………………. mol of water 6.02 x 10 25 molecules of water

0.4 mol of ozone gas ( O3) ………………….x 10 23 molecules of ozone,

contains :

……………………… atoms of Oxygen.

7. Complete these sentences .

a) 1 mol of calcium contains ………………………………………….. atoms

b) 2 mol of iron contains ……………………………………………….. atoms

c) 2 mol of magnesium oxide, (MgO) contains ………………………………………….. ions

d) 2 mol of sodium carbonate, (Na2CO3) contains ……………………………………….

e) 3 mol of carbon dioxide, (CO2) contains …………………………………….. molecules

f) 0.5 mol Copper (II) nitrate, Cu(NO3)2 contains ………………………………….. Cu2+ ions

and …………………………………………………. NO3- ions

C NUMBER OF MOLES AND MASS OF SUBSTANCES

Learning Outcomes
state the meaning of molar mass,
relate molar mass to the Avogadro constant,
relate molar mass of a substance to its relative atomic mass or relative molecular mass,
solve numerical problems to convert the number of moles of a given substance to its
mass and vice versa.


1. The molar mass of a substance
= The molar mass of _________________ mole of the substance.

= The mass of (NA) number of particles

= The mass of ____________________ particles

5


x Molar mass
Number of moles Mass in g
∻ Molar mass

2. Calculating the Mass from a number of Moles

Number of moles = . mass of the substance .
Mass of 1 mole of the substance
Therefore :

Mass of substance = Number of moles x Mass of 1 mole

Example 1 : What is the mass of 2 moles of carbon ?

Mass = 2 x 12
= 24g

Example 2 : What is the mass of 2 moles of H2O ?

Mass = 2 x [ 2(1) + 16 ]

= 36g

3. Calculate the masses of these substances

a) 2 moles of aluminium atoms b) 10 moles of iodine atoms
Mass = Mass =

c) 3 moles of lithium atoms d) 0.5 moles of oxygen gas (O2)
Mass = Mass =

e) 0.1 moles of sodium f) 2 moles of chlorine molecules (Cl2)
Mass = Mass =

g) 1 mole of carbon dioxide ( CO2) h) 3 moles of nitric acid, ( HNO3 )
Mass = Mass =

i) 2 moles of calcium carbonate (CaCO3 ) j) 0.25 moles of calcium chloride (CaCl2 )
Mass = Mass =

6


k) 0.25 moles of sodium hydroxide (NaOH) l) 0.25 moles of sodium carbonate (Na2CO3)
Mass = Mass =

m) 0.5 moles of potassium manganate (VII) n) 0.25 moles of hydrated magnesium sulphate
(KMnO4) (MgSO4.7H2O)
Mass = Mass =

Activity 5

4. Calculate the Number of Moles from a given Mass

Example : How many moles are there in 88g of CO2

Number of moles = 88 = 2 moles
44

a) 2g of helium atoms b) 6g of carbon atoms
Number of moles = Number of moles =

c) 16g of helium atoms d) 4g of sulphur atoms

e) 4g of oxygen molecules (O2) f) 213g of chlorine molecules (Cl2)

g) 0.56g of nitrogen molecules (N2) h) 254g of iodine molecules (I2)

i) 88g of carbon dioxide (CO2) j) 3.1g of sulphur dioxide (SO2)

k) 560g of potassium hydroxide (KOH) l) 392g of sulphuric acid (H2SO4)

7


m) 170g of ammonia (NH3) n) 120g of magnesium oxide (MgO)

o) 4g of sodium hydroxide (NaOH) p) 73g of hydrogen choride (HCl)

q) 15.8g of potassium manganate (VII) r) 8g of ammonium nitrate (NH4NO3)
KMnO4 Number of moles =
Number of moles =

s) 0.78g of aluminium hydroxide Al(OH)3 t) 0.92g of ethanol (C2H5OH)

Activity 6

5. Complete the following table.

Chemical
Element/compound formulae Molar mass Calculate

Copper Cu RAM= 64 (a)Mass of 1 mol = ……………g

(b) Mass of 2 mol = …………. g

(c)Mass of ½ mol = ………….g

(d)Mass of 3.01x1023 Cu atoms

=

Sodium hydroxide NaOH RFM= 40 (a) Mass of 3 mol of sodium hydroxide =

(b) Number of moles of sodium hydroxide in
20 g =

Zinc nitrate Zn(NO3)2 RFM = a) Number of moles in 37.8 g of zinc nitrate :

8


D NUMBER OF MOLES AND VOLUME OF GAS

Learning Outcomes
state the meaning of molar volume of a gas,
relate molar volume of a gas to the Avogadro constant,
make generalization on the molar volume of a gas at a given temperature and
pressure,
calculate the volume of gases at STP or room conditions from the number of moles
and vice versa,
solve numerical problems involving number of particles, number of moles, mass of
substances and volume of gases at STP or room conditions.

Activity 7 (refer text book pg 36, 37 )

1. The molar volume of a gas is defined as the ………………………………………………….

…………………………………………………………….

2. One mole of any gas always has the …………………………………………… under the same

temperature and pressure.

3. The molar volume of any gas is

24 dm3 at ……………………………………………… or

22.4 dm3 at …………………………………………….

Example :

1 mol of oxygen gas, 1 mol of ammonia gas, 1 mol helium gas and 1 mol sulphur dioxide gas occupies
the same volume of 24 dm3 at room condition

x 22.4 / 24 dm3
Number of moles of gas Volume of gas
x 22.4/24 dm3
∻22.4/24 dm3

4. Calculate the volume of gas in the following numbers of moles at STP

Example : Find the volume of 1 mole of CO2 gas

Volume = number of moles x 22.4 dm3
= 1 x 22.4 dm3
= 22.4 dm3

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a) 3 moles of oxygen b) 2 moles of CH4
Volume = Volume =

c) 0.3 moles of Argon d) 0.2 moles of SO3
Volume = Volume =

e) 0.1 moles of N2 f) 1.5 mol of N2
Volume = Volume =

5. Complete the diagram below . (Refer to Page 33,34 & 38-Chemistry textbook)

Volume of gas (dm3)

Mass in gram Number of moles No of particles

Activity 8

Solve these numerical problems

1. What is the volume of 0.3 mole of sulphur dioxide gas at STP?
[Molar volume: 22.4 dm3 mol-1 at STP]

(Ans: 6.72 dm3)

2. Find the number of moles of oxygen gas contained in a sample of 120 cm3 of the gas
at room conditions.
[Molar volume: 24 dm3 mol-1 at room conditions]

10


(Ans: 0.005 mol)

3. Calculate the number of water molecules in 90 g of water, H2O.
[Relative atomic mass: H, 1; O, 16. Avogadro constant, NA: 6.02 x 1023 mol-1]

24
(Ans; 3.01x 10 molecules)

4. What is the volume of 24 g methane ,CH4 at STP?
[Relative atomic mass: H, 1; C, 12. Molar volume: 22.4 dm3 mol-1 at STP]

3
(Ans: 33.6 dm )
5. How many aluminium ions are there in 20.4 g of aluminium oxide, Al2O3?
[Relative atomic mass: O, 16; Al, 27. Avogadro constant, NA: 6.02 x 1023 mol-

23
(2 x 0.2 x 6.02 x10 )

6. Calculate the number of hydrogen molecules contained in 6 dm3 of hydrogen gas at
room conditions.
[Molar volume: 24 dm3 mol-1 at room conditions Avogadro constant, NA: 6.02 x 1023
mol-1]

11


23
(Ans: 1.505x10 molecules)
3 23
7. Find the volume of nitrogen in cm at STP that consists of 2.408 x 10 nitrogen molecules.
[Molar volume: 22.4 dm3 mol-1 at STP. Avogadro constant, NA: 6.02 x 1023 mol-1]

3
(Ans: 8.96 dm )
E CHEMICAL FORMULAE
Learning Outcomes
You should be able to
state the meaning of chemical formula
state the meaning of empirical formula
state the meaning of molecular formula
determine empirical and molecular formula of substances
compare and contrast empirical formula with molecular formula
solve numerical problems involving empirical and molecular formula.
write ionic formula of ions
construct chemical formulaf ionic compounds
state names of chemical compounds using IUPAC nomenclature.
use symbols and chemical formula for easy and systematic communication in the field
of chemistry.

ACTIVITY 9 (Refer text book pg 40)
1) A Chemical formula - A representation of a chemical substance using letters for
……………………………………… and subscripts to show the numbers of each type of
…………………….. that are present in the substance.

The letter H
shows Subscript shows 2
……………. hidrogen atoms in
……………. H2 a molecule

2) Complete this table
Chemical subtance Chemical Notes
formulae

12


Water …………….. 2 atoms of H combine with 1 atom of O

……….. NH3 ……. atoms of H combine with 1 atom of N
Propane
C3H8 …….. atoms of C combine with ……. atoms of
H
Magnesium oxide
…………….. …………………………………………….

……………….. H2SO4 ……………………………………………

3). There are two types of chemical formulae. Complete the following:
** Empirical Formula  The simplest ………… ……….. ratio of atoms of each ……….
in the compound.
** Molecular Formula  The actual …………… of atoms of each …………… that are
present in a molecule of the compound

Molecular formula = (Empirical formula)n
Remember:

Example: (i) Compound – Ethene (ii) Compound – Glucose
Molecular formula - C2 H 4 Molecular formula - C6 H12 O 6

Empirical formula - ................... Empirical formula - ....................
Activity 10
1 Find the empirical formula of a compound
Example of calculation:
a) When 11.95 g of metal X oxide is reduced by hydrogen, 10.35 g of metal X is
produced. Find the empirical formula of metal X oxide [ RAM; X,207; O,16 ]

Element X O
Mass of element(g) 10.35 11.95-10.35
Number of moles of atoms 10.35÷207 (11.95-10.35)÷16

Ratio of moles
Simplest ratio of moles

13


Empirical formula : …………
b) A certain compound contains the following composition:
Na 15.23%, Br 52.98% , O 31.79%, [ RAM : O, 16; Na, 23; Br,80]
(Assume that 100g of substance is used)
Element Na Br O
Mass of element(g) 15.23 52.98 31.79
Number of moles atoms 15.23 ÷23 52.98÷80 31.79÷16

Ratio of moles
Simplest ratio of moles

Empirical formula:: ……………………………………………….

c) Complete the table below.
Compound Molecular Formula Empirical formula Value of n
Water H2O
Carbon Dioxide CO2 CO2
Sulphuric Acid H2SO4
Ethene C2H4 CH2
Benzene C6H6
Glucose C6H12O6

d) 2.52g of a hydrocarbon contains 2.16 g of carbon. The relative molecular mass of the
hydrocarbon is 84. [RAM H,1; C,12]
i. Find the empirical formula of the hydrocarbon

ii. Find the molecular formula of the carbon.

14


Activity 11 :Chemical Formula for ionic compounds:
Complete the table below :
Cation Formula Anion Formula
Hydrogen ion H Flouride ion F
Lithium ion Chloride ion
Sodium ion Bromide ion
Potassium ion Iodide ion
Magnesium ion Hydroxide ion
Calcium ion Ca 2 Nitrate ion

Barium ion Ba 2 Manganate(VII) ion

Copper(II) ion Ethanoate ion CH 3COO
Iron(II) ion O2
Iron (III) ion Sulphate ion
Lead (II) ion Sulphide ion S2
Zinc ion Carbonate ion
Chromium (III) ion Dichromate (VI) ion Cr2O7
2

Aluminium ion Al 3 PO4
3

Ammonium ion Chromate (VI) ion

Avtivity 12
a) Chemical formula of an ionic compound comprising of the ions Xm+ and Yn- is constructed
by exchanging the charges of each element. The formula obtained will XnYm
Example : Sodium oxide Copper (II) nitrate

Na+ O2- Cu2+ NO3-
+1 -2 +2 -1

2 1 1 2

= Na2O = ....................

15


b) Construct a chemical formula for each of the following ionic compounds:
(i) Magnesium chloride (ii) Potassium carbonate

(iii) Calcium sulphate (iv) Copper (II) oxide

(v) Silver nitrate (vi) Zinc nitrate

(vii) Aluminium oxide (viii) Iron(II) hydroxide

(ix) Lead(II) sulphide (x) Chromium(III) sulphate

CHEMICAL EQUATIONS
Learning Outcomes
You should be able to
1. state the meaning of chemical equation
identify the reactants and products of a chemical equation
2. write and balance chemical equations
3. interpret chemical equations quantitatively and qualitatively
4. solve numerical problems using chemical equations
5. identify positive scientific attitudes and values practiced by scientist in doing research
6. justify the need to practice positive scientific attitudes and good values in doing researsh
7. use chemical equations for easy and systematic communication in the field of chemistry.

Activity 13 (refer text book pg 48)
Example: C (s) + O2 (g)  CO 2 (g)
Reactant product
1) Qualitative aspect of chemical equation:
a) Arrow in the equation  the way the reaction is occurring
b) Substances on the left-hand side  ……………………..
c) Substances on the right-hand side  ………………………
d) State of each substance  ………: (s), ………………(l), gas ……….and aqueous solution
……………….

16


2) Quantitative aspect of chemical equations
Coefficients in a balanced equation  the exact proportions of reactants and products in
equation.
Example: 2 H 2 (g) + O2 (g)  2 H 2 O (l)

(Interpreting): 2 molecules (2 mol) of H 2 react with 1 molecule (1 mol) of O2 to produced 2 molecules(2
mol) of water
Complete the following word equations and write in chemical equation

a) Sodium + chlorine  …………………………..
………… + ……………  NaCl
b) Carbon + ………..  Carbon dioxide
………. + …………  ……………………..
c) Sulphur + oxygen  ……………………………
……….. + ………..  …………………………..
d) Zinc + oxygen  ………………………………..
………… + O2  ………………………………..

3) Write a balanced equation for each of the following reactions and interpret the equations
quantitatively.
(a). Carbon monoxide gas + oxygen gas  carbon dioxide gas
………………………………………………………………………………………………………
Interpreting:
……………………………………………………………………………………………………………
(b). Hydrogen gas + nitrogen gas  ammonia gas
……………………………………………………………………………………………………….
Interpreting:
…………………………………………………………………………………………………………..
(c). Aluminium + Iron (III) oxide  aluminium oxide + Iron
……………………………………………………………………………………………………….
Interpreting:
…………………………………………………………………………………………………………….

17


Activity 14
** Numerical Problems Involving Chemical Equations

Hydrogen peroxide decomposes according to the following equation:
2 H 2O2 (l)  2 H 2 O (l) + O2 (g)

1). Calculate the volume of oxygen gas, O2 measured at STP that can be obtained from the

decomposition of 34 g of hydrogen peroxide, H 2O2 .

[Relative atomic mass : H, 1 ; O, 16. Molar volume : 22.4 dm3 m ol 1 at STP]

3
(Ans: 11.2 dm )
2).Silver carbonate Ag2CO3 breaks down easily when heated to produce silver metal
2 Ag2CO3(l) 4 Ag (s) + 2 CO2 (g) + O2
Find the mass of silver carbonate that is required to produce 10 g of silver
[Relative atomic mass: C, 12 ; O, 16 ; Ag, 108]

(Ans : 12.77g)

18


3). 16 g of copper (II) oxide, CuO is reacted with excess methane, CH 4 . Using the equation below, find
the mass of copper that is produced.
[Relative atomic mass : Cu, 64 ; O, 16]
4 CuO (s) + CH 4 (g)  4 Cu (s) + CO2 (g) + 2 H 2 O (l)

(Ans : 12.8 g)

4). A student heats 20 g of calcium carbonate CaCO3 strongly. It decomposes according to the

equation below:
CaCO3 (s) CaO (s) + CO2 (g).
(a). If the carbon dioxide produced is collected at room conditions, what is its volume?

(b). Calculate the mass of calcium oxide, CaO produced.
[Relative atomic mass: C, 12 ; O, 16; Ca, 40. Molar volume :
24 dm3 m ol 1 at room conditions]

3
(Ans : (a). 4.8 dm (b) 11.2 g)

19

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