Chapter 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

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

Activity 2(refer text book pg 29 )

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

Calculating Relative molecular mass,Mr

Mr= The sum of Ar of all atoms present in one molecule

Example:

Mr of Water, H2O = 2(1) + 16 = 18

Mr of Carbon dioxide, CO2 = 12 + 2(16) = 44

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

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

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

of one atom of ……………………………………………

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 ]

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
Zinc nitrate / Zn(NO3)2
Aluminium nitrate / Al(NO3)3
Calcium carbonate / CaCO3
Calcium hydroxide / Ca(OH)2
Hydrated copper(II) sulphate / CuSO4.5H2O / 64 + 32 + 4(16) + 5[2(1) + 16]=250
Hydrated sodium carbonate / Na2CO3.10H2O
Sodium hydrogen sulphate / NaHSO4
Aluminium chloride / AlCl3
Copper(II) sulphate / CuSO4
Zinc carbonate / ZnCO3
Potassium carbonate / K2CO3

[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

You should be able to:

define a mole as the amount of matter that contains as many particles as the

number of atoms in 12 g of 12C,

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.

Activity 3(refer text book pg 30 )

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

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

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

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

atoms, ions, molecules which is 6.02 x 1023

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

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

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

x Avogadro Constant

∻ 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
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)2contains ………………………………….. Cu2+ ions

and …………………………………………………. NO3- ions

C NUMBER OF MOLES AND MASS OF SUBSTANCES

Learning Outcomes

You should be able to:

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.

Activity 4(refer text book pg 33 )

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

x 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
Mass = / b)10 moles of iodine atoms
Mass =
c)3 moles of lithium atoms
Mass = / d)0.5 moles of oxygen gas (O2)
Mass =
e)0.1 moles of sodium
Mass = / f)2 moles of chlorine molecules (Cl2)
Mass =
g)1 mole of carbon dioxide ( CO2)
Mass = / h)3 moles of nitric acid, ( HNO3 )
Mass =
i)2 moles of calcium carbonate (CaCO3 )
Mass = / j)0.25 moles of calcium chloride (CaCl2 )
Mass =
k)0.25 moles of sodium hydroxide (NaOH)
Mass = / l)0.25 moles of sodium carbonate (Na2CO3)
Mass =
m)0.5 moles of potassium manganate (VII) (KMnO4)
Mass = / n)0.25 moles of hydrated magnesium sulphate (MgSO4.7H2O)
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

a)2g of helium atoms
Number of moles = / b)6g of carbon atoms
Number of moles =
c)16g of helium atoms
Number of moles = / d)4g of sulphur atoms
Number of moles =
e)4g of oxygen molecules (O2)
Number of moles = / f)213g of chlorine molecules (Cl2)
Number of moles =
g)0.56g of nitrogen molecules (N2)
Number of moles = / h)254g of iodine molecules (I2)
Number of moles =
i)88g of carbon dioxide (CO2)
Number of moles = / j)3.1g of sulphur dioxide (SO2)
Number of moles =
k)560g of potassium hydroxide (KOH)
Number of moles = / l)392g of sulphuric acid (H2SO4)
Number of moles =
m)170g of ammonia (NH3)
Number of moles = / n)120g of magnesium oxide (MgO)
Number of moles =
o)4g of sodium hydroxide (NaOH)
Number of moles = / p)73g of hydrogen choride (HCl)
Number of moles =
q)15.8g of potassium manganate (VII) KMnO4
Number of moles = / r)8g of ammonium nitrate (NH4NO3)
Number of moles =
s)0.78g of aluminium hydroxide Al(OH)3
Number of moles = / t)0.92g of ethanol (C2H5OH)
Number of moles =

Activity 6

5. Complete the following table.

Element/compound / Chemical
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 :

D NUMBER OF MOLES AND VOLUME OF GAS

Learning Outcomes

You should be able to:

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

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

a)3 moles of oxygen
Volume = / b)2 moles of CH4
Volume =
c)0.3 moles of Argon
Volume = / d)0.2 moles of SO3
Volume =
e)0.1 moles of N2
Volume = / f)1.5 mol of N2
Volume =

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

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]

(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]

(Ans; 3.01x 1024 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]

(Ans: 33.6 dm3)

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-

(2 x 0.2 x 6.02 x1023)

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]

(Ans: 1.505x1023molecules)

7.Find the volume of nitrogen in cm3 at STP that consists of 2.408 x 1023 nitrogen molecules.

[Molar volume: 22.4 dm3 mol-1 at STP. Avogadro constant, NA: 6.02 x 1023 mol-1]

(Ans: 8.96 dm3 )

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.

2) Complete this table

Chemical subtance / Chemical formulae / Notes
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

Remember:

Example: (i) Compound – Ethene(ii) Compound – Glucose

Molecular formula - Molecular formula -

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 oxideis 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

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

Find the molecular formula of the carbon.

Activity 11 :Chemical Formula for ionic compounds:

Complete the table below :

Cation / Formula / Anion / Formula
Hydrogen ion / / Flouride ion /
Lithium ion / Chloride ion
Sodium ion / Bromide ion
Potassium ion / Iodide ion
Magnesium ion / Hydroxide ion
Calcium ion / / Nitrate ion
Barium ion / / Manganate(VII) ion
Copper(II) ion / Ethanoate ion /
Iron(II) ion /
Iron (III) ion / Sulphate ion
Lead (II) ion / Sulphide ion /
Zinc ion / Carbonate ion
Chromium (III) ion / Dichromate (VI) ion /
Aluminium ion / /
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 = ......

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

state the meaning of chemical equation

identify the reactants and products of a chemical equation

write and balance chemical equations
interpret chemical equations quantitatively and qualitatively
solve numerical problems using chemical equations
identify positive scientific attitudes and values practiced by scientist in doing research
justify the need to practice positive scientific attitudes and good values in doing researsh
use chemical equations for easy and systematic communication in the field of chemistry.

Activity 13 (refer text book pg 48)

Example:C (s) + (g) (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 ……………….

2) Quantitativeaspect of chemical equations

Coefficients in a balanced equation  the exact proportions of reactants and products in

equation.

Example:2(g) + (g) 2 (l)

(Interpreting): 2 molecules (2 mol) of react with1 molecule (1 mol) ofto 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:

…………………………………………………………………………………………………………….

Activity 14

** Numerical Problems Involving Chemical Equations

Hydrogen peroxide decomposes according to the following equation:

2 (l)  2 (l) + (g)

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

decomposition of 34 g of hydrogen peroxide, .

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

(Ans: 11.2 dm3)

2).Silver carbonate Ag2CO3 breaks down easily when heated to produce silver metal

2 Ag2CO3(l) 4(s) + 2 (g) +

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)

3). 16 g of copper (II) oxide, is reacted with excess methane, . Using the equation below, find the mass of copper that is produced.

[Relative atomic mass : Cu, 64 ; O, 16]

4 (s) + (g)  4 (s) + (g) + 2 (l)

(Ans : 12.8 g)

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

equation below:

(s) (s) + (g).

(a). If the carbon dioxide produced is collected at room conditions, what is its volume?

(b). Calculate the mass of calcium oxide, produced.

[Relative atomic mass: C, 12 ; O, 16; Ca, 40. Molar volume :

24 dm3 at room conditions]

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