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