Page 8 - Focus SPM KSSM F4 2020 - Chemistry

Chemistry Form 4 Chapter 3 The Mole Concept, Chemical Formula and Equation
3. Using the knowledge about relative mass, we Q2 The mass of a sulphur atom is 8 times more than
can solve various calculation problems. See the mass of a helium atom. What is the relative
Example 3.3. atomic mass of sulphur?
[Relative atomic mass: He, 4]
EXAMPLE 3.3 Q3 Calculate the relative molecular mass of the
following substances:
The relative formula mass of X SO is 142. (a) Phosphorus, P 4
2
4
Calculate the relative atomic mass of element X. (b) Carbon monoxide gas, CO
[Relative atomic mass: O, 16; S, 32] (c) Sucrose, C H O 11
12
22
(d) Benzoic acid, C H COOH
6
5
Solution [Relative atomic mass: H, 1; C, 12; O, 16; P,
RAM of X = x 31]
Given RFM of X SO = 142 Q4 Calculate the relative formula mass of the
2
4
2(x) + 32 + 4(16) = 142 following substances:
2x + 96 = 142 (a) Sodium oxide, Na O
2
2x = 142 – 96 = 46 (b) Zinc nitrate, Zn(NO )
3 2
x = 46 = 23 (c) Potassium thiosulphate, K S O 3
2
2
2
Chapter
(d) Hydrated sodium carbonate, Na CO .10H O
3 RAM of X is 23. [Relative atomic mass: H,1; C,12; N,14; O,
2
3
2
16; Na, 23; S, 32; K, 39; Zn, 65]
Checkpoint 3.1 Q5 Element Z forms a chloride salt with the formula
ZCI and relative formula mass of 95. What is
2
Q1 How many oxygen atoms have the same mass the relative atomic mass of element Z?
as three copper atoms? [Relative atomic mass: Cl, 35.5]
[Relative atomic mass: C, 12; Cu, 64]
3.2 Mole Concept
1. Chemists use the ‘mole’ unit to represent the quantity of a substance.
One mole of substance is
defined as the quantity of
substance containing the
same number of atoms Avogadro’s constant, N is defined
contained in 12 g of as the number of particles.
A
carbon-12, which is 6.02 × (N = 6.02 × 10 mol )
-1
23
10 particles A
23
2. The number of particles per mole, which is 6.02 × 10 is determined experimentally and is known as
23
Avogadro’s constant or Avogadro’s number.
3. The way of using the mole unit is exactly the same as that of using the dozen unit. See Figure 3.3.

1 dozen of pencils = 12 units of pencils 2 dozen of pencils = 24 units of pencils

Contains Contains
6.02 x 10 23 2 x 6.02 x 10 23
molecules molecules
of water of water

Figure 3.3 The mole unit is used in the same way as the dozen unit

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