4S1
Thursday, 24/2/2011
7.30 – 8.40 am
Previously…
Relationship of Mass and Number of
Moles
Number of moles =
Mass
Molar mass
Mass = Number of moles x Molar mass
Number of
moles (mol)
× Molar mass
(g mol-1)
÷ Molar mass
(g mol-1)
Mass
(g)
• 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 gas
Thus, the molar volume is
also the volume occupied
23
by 6.02×10 particles of
gas
Molar volume of any
gas is:
3
-1
• 22.4 dm mol at STP
OR
3
-1
• 24 dm mol at room
conditions
Refer Textbook
page 37
Do correction
for the unit of
molar volume
Relationship of Volume of Gas and Number
of Moles
Number of moles = Volume of gas
Molar volume
Volume of gas = Number of moles x Molar
volume
Number of
moles (mol)
× Molar volume
(dm3 mol-1)
÷ Molar volume
(dm3 mol-1)
Volume of
gas (dm3)
REMINDER!!!
SI unit of volume is
&
3
3
1 dm = 1000 cm
3
dm
Example 1
Find 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 2
Find 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 3
Find 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
3
= 33.6 dm
Example 4
Find 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 1
Find 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 2
Find 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
3
= 21.6 dm