Page 10 - Focus SPM KSSM F4 2020 - Chemistry

Chemistry Form 4 Chapter 3 The Mole Concept, Chemical Formula and Equation
Table 3.3 Molar mass of a few substances
EXAMPLE 3.5
Molar
mass
How many moles of carbon dioxide contain Substance Relative mass (g mol )
-1
1.806 × 10 CO molecules?
23
2 Atomic Magnesium, Mg RAM = 24 24
Solution substance Zinc, Zn RAM = 65 65
The number of moles of carbon dioxide Hydrogen gas,
= Number of particles ÷ N A Molecular H 2 RMM = 2(1) = 2 2
1.806 × 10 23 substance RMM = 12 + 4(1)
= Methane, CH 4 = 16 16
6.02 × 10 23
= 0.3 mol Zinc bromide, RFM = 65 + 2(80) 225
Ionic ZnBr 2 = 225
substance Sodium sulphate, RFM = 2(23) + 32 142
EXAMPLE 3.6 Na SO 4 + 4(16) = 142
2
[Relative atomic mass : H,1; C,12; O,16; Na, 23;
0.5 mole of magnesium chloride, MgCl are Mg, 24; S, 32; Zn, 65; Br, 80]
2
dissolved in a beaker of water. How many ions
Chapter
3 are present in the beaker?
Solution
Magnesium chloride, MgCl is an ionic
2
substance. So, 1 mole of magnesium chloride Figure 3.4 Molar mass of magnesium is 24 g mol -1
23
has 6.02 × 10 MgCl units.
2
The number of MgCl units in 0.5 mole of EXAMPLE 3.7
2
magnesium chloride What is the mass of 1.2 moles of sodium
= number of moles × N A hydroxide, NaOH?
= 0.5 × 6.02 × 10 units [Relative atomic mass: H, 1; O, 16; Na, 23]
23
Each MgCl unit is made up of three ions,
2
which are one magnesium ion and two chloride Solution
ions. So, the number of ions in the beaker RFM of sodium hydroxide, NaOH
= 3 × number of MgCl 2 = 23 + 16 + 1 = 40
= 3 × 0.5 × 6.02 × 10 ions So, the molar mass of sodium hydroxide
23
= 9.03 × 10 ions = 40 g mol -1
23
The mass of 1.2 moles of sodium hydroxide
= number of moles × molar mass
Number of moles and mass of substance = 1.2 × 40 = 48 g
1. Chemists measure the number of moles of
substance by just weighing its mass. Mass of EXAMPLE 3.8
substance and number of moles are linked by
molar mass as shown below. How many moles of molecules are there in 450
g of water, H O?
2
Number × molar mass Mass in [Relative atomic mass: H, 1; O, 16]
of moles ÷ molar mass grams Solution
RMM of water, H O = 2(1) + 16 = 18
2
2. Molar mass of a substance is the mass of one So, the molar mass of water = 18 g mol -1
mole of the substance in grams. The unit for The number of moles of molecules in 450 g of
water
molar mass is gram per mole (g mol ).
-1
= mass ÷ molar mass
3. The value of molar mass is the same as the = 450 = 25 mol
relative mass of the substance. 18

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