TOPIC: AVOGADRO’S CONSTANT
AND MOLAR GAS VOLUME
OBJECTIVES:
Students should be able to:
1. Understand that one mole of a substance is 6 × 1023 particles
which is Avogadro’s constant
2. Solve problems involving Avogadro’s constant
3. Explain Molar gas volume
4. Solve problems involving molar gas volume
AVOGADRO’S CONSTANT
One mole of a substance contains 6.02 × 1023 particles. Particles can
either be atoms, ions or molecules.
This number is known as Avogadro constant.
Therefore, an amount of substance containing 6 × 1023 particles is
called a mole.
EXAMPLES
1. Find the number of atoms in 11.5g of sodium atom. (Na = 23)
2. Calculate the number of
a. hydrogen molecules
b. hydrogen atoms;
in 5g of hydrogen gas.
(H=1)
3. How many hydrogen ions are in 4.9g H2SO4(aq). (H=1, S=32, O=16)
FURTHER EXAMPLES
• 1. a. Find the number of oxygen molecules in 3.2g of Oxygen gas.
• b. Calculate the number oxygen atoms in the 3.2g of oxygen gas.
(O = 16)
• 2. How many chloride ions are there in 0.2 moles of Calcium chloride?
(Ca=40; Cl=35.5)
MOLAR GAS VOLUME
Molar gas volume is taken as 24dm3 at room temperature and
pressure` (r.t.p.).
𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑔𝑎𝑠 𝑖𝑛 𝑑𝑚3 𝑎𝑡 𝑟𝑡𝑝
Number of moles 𝑜𝑓 𝑎 𝑔𝑎𝑠 =
24𝑑𝑚3
Also,
Volume 𝑜𝑓 𝑎 𝑔𝑎𝑠 = Number of moles 𝑜𝑓 𝑎 𝑔𝑎𝑠 × 24𝑑𝑚3
EXAMPLE
1. Calculate the volume of carbon dioxide gas, (CO2)
occupied by:
a. 5 moles
b. 0.5 mole;
of the gas measured at rtp.
2. Calculate the number of moles of ammonia gas, (NH3) in
a volume of 72 dm3 of the gas measured at rtp.
Ans: 1a.
= 120dm3
b.
= 12dm3
2.
= 3 mol
ASSIGNMENT
• 1. Calculate the mass of 0.5 mole of:
(i) magnesium nitrate
(ii) ammonia.
(Mg = 24; N = 14; O = 16; H = 1)
2. Calculate the volume occupied, at rtp, by the following gases.
(One mole of any gas occupies a volume of 24dm3 at rtp.)
a. 12.5 moles of sulfur dioxide gas.
b. 0.15 mole of nitrogen gas.
(S=32, O=16, N=14)
3. a. Sodium chloride is made up of Na+ and Cl- ions. a How many sodium
ions are there in 58.5 g of sodium chloride? (Ar: Na=23; Cl=35.5.)
• b. Find the number of ammonia molecules in ammonia gas.
(N = 14; H = 1)