1. Atoms, Molecules, and Stoichiometry
Relative masses;
The mole and related quantities

2. Relative isotopic mass5
Relative Atomic Mass
Example 2: The abundance of the isotopes of chlorine are: Calculate the relative atomic mass of chlorine
Isotopes
Relative isotopic mass
% abundance
35Cl
34.97
75.53
37Cl
36.97
24.47
= 35.46

3. What will I learn? What is relative molecular / formula mass?
How to calculate relative molecular / formula mass?
What is the mole?
How is the mole related to molar mass and molar volume?

4. Relative Molecular Mass (Mr)5
Relative Molecular Mass (Mr)
Definition: The average mass of one molecule compared to the mass of 12C atom.
For calculations, the relative molecular mass of a molecule, is equal to the sum of the Ar of the atoms that make up the molecule.
Mr =  (Ar)
Relative mass of a particle
average mass of one molecule
x mass of one 12C atom =
All atoms

5. Relative Molecular Mass (Mr)
Examples: Calculate the Mr of H2O and SO2 using the Ar given in the Periodic Table.
Mr(H2O) = 2 x Ar(H) + Ar(O) =
= 18.0
Mr(SO2) = Ar(S) + 2 x Ar(O)
= (16.0)
= 64.1

6. Relative Formula Mass (Mr)6
Relative Formula Mass (Mr)
Definition: (for ionic compounds only) The average mass of one formula unit compared to the mass of 12C atom.
For calculations, the relative formula mass of an ionic compound, is equal to the sum of the Ar of the atoms shown in the formula of the ionic compound.
Mr =  (Ar)
Relative mass of a particle
average mass of one formula unit
x mass of one 12C atom =
All atoms

7. Relative Formula Mass (Mr)
Examples: Calculate the Mr of NaCl and (NH4)2CO3 using the Ar given in the Periodic Table.
Mr(NaCl) = Ar(Na) + Ar(Cl) =
= 58.5
Mr{(NH4)2CO3} = 2{Ar(N) + 4Ar(H)} + Ar(C) + 3Ar(O)
= 2( ) (16.0)
= 96.0

8. The Mole (n)
A counting unit – representing a certain number of something
Examples:
One dozen of roses (12 stalks)
One pack of tissue (10 sheets)
One ream of printing paper (500 sheets)
One mole of particles
(6.02 x 1023 particles)
Avogadro constant

9. The Mole (n)
Definition: A mole is the amount of substance that contains Avogadro’s number of particles
Or A mole is the amount of substance that contains the same number of particles as there are atoms in 12.0 g of 12C.
 12.0 g of 12C contains exactly 6.02 x 1023 atoms!!

10. The Mole (n) Example: 1 mol of H2O would contain:
6.02 x 1023 molecules
1 mole of O atoms

11. The Mole (n) Example: 1 mol of H2O would contain:
6.02 x 1023 molecules
1 mole of O atoms (6.02 x 1023 O atoms)

12. The Mole (n) Example: 1 mol of H2O would contain:
6.02 x 1023 molecules
1 mole of O atoms (6.02 x 1023 O atoms)
2 moles of H atoms

13. The Mole (n) Example: 1 mol of H2O would contain:
6.02 x 1023 molecules
1 mole of O atoms (6.02 x 1023 O atoms)
2 moles of H atoms (2 x 6.02 x 1023 H atoms)
3 moles of atoms (in total)

14. The Mole (n) Example: 1 mol of H2O would contain:
6.02 x 1023 molecules
1 mole of O atoms (6.02 x 1023 O atoms)
2 moles of H atoms (2 x 6.02 x 1023 H atoms)
3 moles of atoms (in total) (3 x 6.02 x 1023 atoms)

15. 7
Molar mass (M)
Chemical reactions are normally carried out using known masses
Hence, need to relate mass to amount for a substance

16. Molar mass (M)
Chemical reactions are normally carried out using known masses
Hence, need to relate mass to number of moles for a substance
Definition: The molar mass of a substance is the mass of one mole of that substance
The molar mass of a substance has the same numerical value as the Ar or Mr of that substance
Except molar mass has unit of g mol-1

17. Molar mass (M)
Example:
Molar mass of Fe =

18. Molar mass (M) Example: Molar mass of Fe = 55.8 g mol-1
Molar mass of H2O =

19. Molar mass (M) Example: Molar mass of Fe = 55.8 g mol-1
Molar mass of H2O = 18.0 g mol-1
Molar mass of MgCl2 =

20. Molar mass (M) Example: Molar mass of Fe = 55.8 g mol-1
Molar mass of H2O = 18.0 g mol-1
Molar mass of MgCl2 =

21. Molar mass (M) Example: Molar mass of Fe = 55.8 g mol-1
Molar mass of H2O = 18.0 g mol-1
Molar mass of MgCl2 = 95.3 g mol-1

22. Molar volume (Vm) For gases, it is more convenient to measure volume
Hence we need to relate volume with amount of a substance

23. Molar volume (Vm) For gases, it is more convenient to measure volume
Hence we need to relate volume with number of moles of a substance
Definition: The molar volume of a gas is the volume occupied by one mole of gas.

24. Molar volume (Vm)
Since volume is dependent on pressure and temperature, we define two conditions:
Standard pressure and temperature (s.t.p.) temperature: 273K, pressure: 1 atm
Room pressure and temperature (r.t.p.) temperature: 298K, pressure: 1 atm
The molar volume of any gas at s.t.p. = 22.4 dm3
The molar volume of any gas at r.t.p. = 24.0 dm3

25. Molar volume (Vm) Example:
Volume of 1 mole of CO2 at s.t.p. = 22.4 dm3
Volume of 1 mole of CO2 at r.t.p. = 24.0 dm3
Volume of 1 mole of N2 at s.t.p. = 22.4 dm3
Volume of 1 mole of N2 at r.t.p. = 24.0 dm3

26. Why are they important??? The mole Avogadro constant Molar mass8
Why are they important???
The mole
Avogadro constant
Molar mass
Molar volume

27. number of particles (N)
Why are they important???
mass (m)
Related by molar mass
number of particles (N)
number of moles (n)
Related by Avogadro constant
Related by molar volume
volume (V)

28. number of particles (N)
Why are they important???
mass (m)
number of particles (N)
number of moles (n)
volume (V)

29. Conversion between various quantities
Example 1: In 4.0 g of oxygen gas, how many oxygen atoms are there?

30. number of particles (N)
Why are they important???
mass (m)
number of particles (N)
number of moles (n)
volume (V)

31. Conversion between various quantities
Example 1: In 4.0 g of oxygen gas, how many oxygen atoms are there?

32. Conversion between various quantities
Example 1: In 4.0 g of oxygen gas, how many oxygen atoms are there?
Since

33. Conversion between various quantities
Example 1: In 4.0 g of oxygen gas, how many oxygen atoms are there?

34. Conversion between various quantities
Example 1: In 4.0 g of oxygen gas, how many oxygen atoms are there?

35. Conversion between various quantities
Example 1: In 4.0 g of oxygen gas, how many oxygen atoms are there?
There are 1.51 x 1023 oxygen atoms in 4.0 g of oxygen

36. Conversion between various quantities
Example 2: What is the mass of 300 cm3 of sulphur dioxide measured at r.t.p.?

37. number of particles (N)
Why are they important???
mass (m)
number of particles (N)
number of moles (n)
volume (V)

38. Conversion between various quantities
Example 2: What is the mass of 300 cm3 of sulphur dioxide measured at r.t.p.?

39. Conversion between various quantities
Example 2: What is the mass of 300 cm3 of sulphur dioxide measured at r.t.p.?

40. Conversion between various quantities
Example 2: What is the mass of 300 cm3 of sulphur dioxide measured at r.t.p.?

41. Conversion between various quantities
Example 2: What is the mass of 300 cm3 of sulphur dioxide measured at r.t.p.?

42. Conversion between various quantities
Example 2: What is the mass of 300 cm3 of sulphur dioxide measured at r.t.p.?

43. Conversion between various quantities
Example 2: What is the mass of 300 cm3 of sulphur dioxide measured at r.t.p.?

44. Conversion between various quantities
Example 2: What is the mass of 300 cm3 of sulphur dioxide measured at r.t.p.?
The mass of sulphur dioxide is g

45. What will I learn? What is relative molecular / formula mass?
How to calculate relative molecular / formula mass?
What is the mole?
How is the mole related to molar mass and molar volume?

46. “If you can’t win, make the person infront break the record” Anonymous
End of Lecture 2
“If you can’t win, make the person infront break the record” Anonymous