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chapter 3

CHEMICALFORMULAE AND

EQUATIONS



RELATIVE ATOMIC MASS

y The relative atomic mass of an element is the

average mass of one atom of the element when

compared with 1/12 of the mass of an atom of carbon-12.

y Relative atomic mass does not have any unit

y Example 1

an atom of oxygen is 16 times heavier than 1/12 of

the mass of an atom of carbon-12. Its relative atomic

mass is 16.



y Example 2

the relative atomic mass of Mg is 24 and the relativeatomic mass of carbon-12 is 12. How many times is

one atom of Mg heavier than one atom of carbon-12

answer :

the number of times one atom of Mg is heavier thanone atom of carbon-12

= the R.A.M of Mg

the R.A.M of carbon-12

= 24

12

= 2



RELATIVE MOLECULAR MASSy The relative molecular mass of a molecule is the

average mass of the molecule when compared with

1/12 of the mass of one atom of carbon-12y Example 3

the relative molecular mass of a molecule of

ammonia is 17. this means that one molecule of

ammonia is 17 times heavier than 1/12 of one atomof carbon-12



Why Carbon-12 is used as a

standard1) Its mass can be more easily measured with a mass

spectrometer

2) Carbon-12 is easily found as its compounds areabundant

y Example 4

Calculate the relative molecular mass of carbon dioxide.

1 molecule of CO2 consists of 1 atom of carbon and 2

atom of oxygen.relative atomic mass of 2 atoms oxygen = 2 x 16

relative atomic mass of 1 atom of carbon = 1 x 12

relative molecular mass of CO2 = 32+12 = 44



Calculation of Relative Molecular

Massy Step 1 : determine the molecular formula

y Step 2 : find the relative atomic mass of each

element in the moleculey Step 3 : add up all the relative atomic masses of the

element or elements

Molecule Molecular formula Relative molecular

mass

Chlorine Cl2 2 x 35.5 = 72

Ammonia NH3 14 + (3 x 1) = 17



THE MOLE AND THE NUMBER OF

PARTICLEy Pairs and dozen are examples of units that we use

y In chemistry, we use the unit mole to measure theamount of substances

yThe symbol of mole is mol

y Example 5

a) 1 pair of shoes = 2 shoes

b) 1 dozen eggs = 12 eggs

c) 1 mole of carbon atoms = 6.02 x 1023 atoms

y A mole is an amount of substance that contains as manyparticles as the number of atoms in exactly 12 g of carbon-12

y 1 mol of carbon-12 contains 6.02 x 1023 carbon atoms



Avogadro constant, N A

y Is the number of particles in one mole of a substances

Mass, g

Volume ,cm3 / dm3

Number of Particle

MOLE

÷ 6.02 X 1023

× 6.02 X 1023

÷ Mr

× Mr

÷ 24 dm3 or 22.4 dm3× 24 dm3 or 22.4 dm3



To determine the number of moles

from the mass of a substance

(converting mass to mole)y Number of mole atoms =

y Example 6how many moles of matter are there in

(a) 5 g of nitrogen

(b) 80 g of carbon

[relative atomic mass : N,14 ; C,12]

answer

(a) 0.357 mol (b) 6.67 mol



To determine the mass from the

number of moles (changing mole to

mass)y Mass = number of moles x relative atomic mass

y Example 7

how many grams of matter are there in(a) 2 moles of nitrogen

(b) 6 moles of carbon

[relative atomic mass : N,14 ; C,12]

answer

(a) 28 g (b) 72 g



To determine the number of

particles from the number of

molesy Number of particle = no. of moles x 6.02x1023

y Example 8

the relative atomic mass of nitrogen is 14. calculate

(a) the number of moles of nitrogen in 2 g of nitrogen

(b) the number of atoms in 2 g of nitrogen

answer :(a) 0.143 mol

(b) 8.609x1022 nitrogen atoms



To determine the number of moles

from the number of atomsy Number of moles of atoms =

y Example 9

how many moles are there in :(a) 12.04x1023 atoms of chlorine?

(b) 1.02x1046 atoms of sodium?

answer

(a) 2 mol

(b) 1.69x1022 mol



The mole and the mass of

substancesy The molar mass of a substances is the mass of 1

mole of the substance

y The molar mass of an element is the relative atomic

mass of the element expressed in grams per mole

y Example 10

the molar mass of sodium

= the mass of 1 mole of sodium= the relative atomic mass of sodium expressed in

grams per mole

= 23 g mol-1

23 g of Na contain 6.02x1023 atoms of sodium



y

The molar mass of a compound is the relativemolecular mass of the compound expressed in

grams per mole

y Example 11

the molar mass of SO2 gas= the mass of 1 mole of sulphur dioxide

= the relative molecular mass of sulphur dioxide

expressed in grams per mole

= 32 + (2 x 16)

= 64 g mol-1

64 g of SO2 contain 6.02x1023 molecules of SO2



To determine the Molar Mass from

the Number of Molesy Example 12

what is the molar mass of

(a) 4 moles of H atoms ?

(b) 0.5 moles of Na atoms ?[ H, 1 ; Na, 23 ]

answer :

(a) 4 g (b) 12.5 g

y Example 13how many moles are there in 16 g of copper(II) oxide

CuO ? [ Cu, 64 ; O,16 ]

answer

= 0.2 mol



y Example 14

find the mass of 0.25 moles of sulphuric acid H2

SO4

.

[ H,1 ; S,32 ; O,16 ]

answer

24.5 g

y

Example 15Calculate the mass of the following ionic compounds

(a) 0.5 mole of NaCl

(b) 0.1 mole of KNO3

[ O,16 ; Na,23 ; Cl,35.5 ; N,14 ; K,39 ]

answer

(a) 29.25 g

(b) 10.1 g



The Mole and the Volume of Gasy

The molar volume of a gas is the volume occupiedby one mole of gas

y The molar volume of any gas is 22.4 dm3 at STP or

24 dm3 at room condition (r.t.p)

y A fixed volume of any gas always contains the samenumber of molecules.

y Avogadro¶s Law

= equal volumes of all gases contain the same

number of molecules under the same temperatureand pressure

y From this law, we can conclude that 1 mole of any

gas has the same volume. (under the same

temperature and pressure)



To determine the volume of a gas

from the number of molesy Example 16

calculate the volumes of the following quantities of

gas at STP

(a) 3.2 x 10-3 moles of CO2

answer

(a) 0.0717 dm3

yExample 17what is the volume of 0.08 moles of gas at room

temperature and pressure ?

answer

1.92 dm3



To determine the volume of a gas

from its massy Example 18

a container has 10 g of oxygen

(a) how many moles of O2 gas are there in the

container

(b) how many molecules of O2 gas are there in the

container

(c) what is the volume of O2

gas at room temperature

and pressure

answer :

(a) 0.3125 mol (b) 1.881x1023

(c) 7.5 dm3



To determine the mass of a gas

from its volumey Example 19

a container was filled with 12dm3 of SO2 gas at room

temperature and pressure. Find(a) the number of moles of SO2 present in the

container

(b) the mass of the sulphur dioxide gas

[ S,32 ; O,16 ]

answer :

(a) 0.5 mol

(b) 32 g



To determine the number of particle

of a gas from the number of moles

y Example 20

how many particles of gas are there in 22 g of

carbon dioxide ?

answer :

3.01 x 1023



To determine the number of

particles of a gas from the volumey Example 21

a container contains 15 g of hydrogen. Calculate

(a) its volume at room temperature and pressure(b) the number of molecules present

(c) the number of atoms present

answer :

(a) 180 dm3

(b) 4.52 x 1024 molecules

(c) 9.04 x 1024 atoms



y Example 22

find the mass of the following gases :(a) 1 mole of SO2

(b) 2.5 moles of Br 2 gas

[ Br,80 ; O,16 ]

answer :

(a) 64 g (b) 400 g

y Example 23

how many atoms are there in 4.7 moles of calcium ?

[ Ca,40 ]

answer :

2.829 x 1024



y Example 24

a balloon is filled with 9.0 dm3 of CO2 at roomtemperature and pressure of 1 atmosphere.

(a) how many atoms of CO2 does it contain ?

(b) will answer be different if the balloon is filled with

the same volume of helium ?answer :

(a) 6.773 x 1023

(b) the answer will still the same.Why ?

this is because all gases at the same temperature

and pressure will have the same volume

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