chapter 3: chemical formulae & equations
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4S1Thursday, 24/2/2011
7.30 – 8.40 am
Previously…
Relationship of Mass and Number of MolesNumber of moles = Mass
Molar massMass = Number of moles x Molar mass
Number of moles (mol)
Mass (g)
× Molar mass
(g mol-1)
÷ Molar mass
(g mol-1)
• Chemists determines the number of moles of any gas by measuring its volume as gases cannot be weighed• It was found that under the same temperature & pressure, equal volumes of all gases contain the same number of particles. Therefore, the concept of molar volume was introduced
Molar volume = the volume occupied by one mole of gasThus, the molar volume is also the volume occupied by 6.02×1023
particles of gas
Molar volume of any gas is:
• 22.4 dm3 mol-1 at STP
OR• 24 dm3 mol-1 at room conditions
Refer Textbook page 37
Do correction for the unit
of molar volume
Relationship of Volume of Gas and Number of MolesNumber of moles = Volume of gas
Molar volumeVolume of gas = Number of moles x Molar
volumeNumber of moles
(mol)
Volume of gas (dm3)
× Molar volume
(dm3 mol-1)÷ Molar volume
(dm3 mol-1)
REMINDER!!!SI unit of volume is dm3
&1 dm3 = 1000 cm3
Example 1Find the number of moles of 6.72 dm3 of O2 at STP
Number of moles of O2 gas= volume of O2 gas molar volume at STP= 6.72 dm3 22.4 dm3 mol-1
= 0.3 mol
Example 2Find the number of moles of 560 cm3 of CO2 at room condition
Number of moles of CO2 gas= volume of CO2 gas molar volume at room condition= 0.56 dm3 24 dm3 mol-1
= 0.0233 mol
Example 3Find the volume 1.5 mol of H2 gas at STP
Volume of H2 gas= Number of moles of H2 gas × Molar volume at STP= 1.5 mol × 22.4 dm3 mol-1
= 33.6 dm3
Example 4Find the volume 0.4 mol of He gas at room conditions
Volume of He gas= Number of moles of He gas × Molar volume at room condition= 0.4 mol × 24 dm3 mol-1
= 9.6 dm3
Exercise 1Find the number of moles of 896 cm3 of HCl gas at STP
Number of moles of HCl gas= volume of HCl gas molar volume at STP= 0.896 dm3 22.4 dm3 mol-1
= 0.04 mol
Exercise 2Find the volume 0.9 mol of N2 gas at room temperature
Volume of N2 gas= Number of moles of N2 gas × Molar volume at RT= 0.9 mol × 24 dm3 mol-1
= 21.6 dm3
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