1. Molecular mass and formula mass
Foundation Year
Molecular mass and formula mass 2
Dr. Marwa Eid

2. Molecular Mass Objectives
-After this Lecture the students will be able to convert between:
-Moles and grams
-Moles and particles

3. Molecular Mass “MM”
Molecular Mass is conversion factor between grams and moles.
Number of grams in 1 mole
(n= mass/MM MM=mass/n)
MM= # Protons + # Neutrons in a nucleus.
= atomic mass

4. Molecular Mass 1 mole of Mg atoms = 24.3 g 1 mole of Cu atoms = 63.5 g
1 mole of C atoms = g
1 mole of Mg atoms = 24.3 g
1 mole of Cu atoms = 63.5 g

5. Molecular Mass

6. Molecular Mass Learning Check
Give the molar mass for each (to the tenths decimal place).
A. 1 mole K atoms = ________
B. 1 mol Sn atoms = ________

8. Molecular Mass Solution
Give the molar mass for each (to the tenths decimal place).
A. 1 mole K atoms = 39.1 g
B. 1 mole Sn atoms = g

9. Moles and Grams I mole of Na = 22.99 g 3.5 mole (Na)= ??? g
What is the mass of 3.5 moles of sodium (Na)?
I mole of Na = g
3.5 mole (Na)= ??? g
Mass = (3.5* 22.99)/1
Mass = 80.5 g

10. Moles and Grams I mole of Fe = 55.85 g ??? mole (Na)= 25 g
How many moles of iron (Fe) does 25 g of Fe represents?
I mole of Fe = g
??? mole (Na)= 25 g
# moles = (25*1 )/ 55.85
# moles (n) = 0.448

11. One Mole a Compound 32.1 g 55.9 g 58.5 g 294.2 g 342.2 g
One-Mole Quantities

12. Molecular Mass of a Compound
“The molecular mass of a compound is the sum of the molecular mass of the elements in the formula”
1 mole of methane
1 mole of carbon and 4 moles of hydrogen
The molar mass of (“one mole of”) a compound is obtained by summing the masses of the appropriate number of moles of component atoms.

13. Molecular Mass of a Compound

14. Molecular Mass of a Compound
Example
Calculate the molar mass of CaCl2.
Solution
Element
Number of Moles (n)
Molecular Mass
(MM)
Total Mass
= MM*n Ca 1
40.1 g/mole
40.1 *1= 40.1 g Cl 2
35.5 g/mole
35.5*2= 71.0 g
CaCl2
= g

15. Molecular Mass of a Compound
Calculate the molar mass of K3PO4.
Element
Number of Moles (n)
Molecular Mass
(MM)
Total Mass
= MM * n K 3
39.1 g/mole
117.3*3= g P 1
31 g/mole
31*1 = g O 4
16 g/mole
16 * 4 = 64.0 g
K3PO4
212.3 g

16. Molecular Mass of a Compound
Calculate the molar mass of N2O4.
Element
Number of Moles (n)
Molecular Mass
(MM)
Total Mass
= MM * n N 2
14 g/mole
14*2= 28 g O 4
16 g/mole
16*4 = 64 g
N2O4
92 g

17. Molecular Mass of a Compound
Calculate the molar mass of H2SO4.
Element
Number of Moles (n)
Molecular Mass
(MM)
Total Mass
= MM * n H 2
1.008 g/mole
1.008*2 = g S 1
32.07 g/mole
32.07*1 = g O 4
16 g/mole
16*4 = 64.0 g
K3PO4
98.09 g

18. Molecular Mass Learning Check
What is the molar mass of each of the following?
A. K2O
B. Al(OH)3

19. Molecular Mass Solution A. K2O 94.2 g/mole
2 mole K (39.1 g/mole) + 1 mole O (16.0 g/mole)
78.2 g g = g
B. Al(OH) g/mole
1 mole Al (27.0 g/mole) + 3 mol O (16.0 g/mole)
+ 3 mol H (1.01 g/mole)
27.0 g g g = g

20. Moles and Grams Solution
Aluminum is often used for the structure of light-weight bicycle frames. How many grams of Al are in 3.00 moles of Al? moles Al ? g Al
Solution

21. 1 mole= 6.02 X 1023 particles Avogadro’s Number
Avogadro’s number is a conversion factor between moles and particles

22. Moles and Particles 1 dozen cookies = 12 cookies
1 mole of cookies = 6.02 X 1023 cookies
1 dozen cars = 12 cars
1 mole of cars = 6.02 X 1023 cars
1 dozen Al atoms = 12 Al atoms
1 mole of Al atoms = 6.02 X 1023 atoms
Note that the NUMBER is always the same, but the MASS is very different!
Mole is abbreviated mol (gee, that’s a lot quicker to write, huh?)

23. A Mole of Particles Contains 6.02 x 1023 Atom
= x 1023 C atoms
= x 1023 H2O molecules
= x 1023 NaCl “molecules”
(technically, ionics are compounds not molecules so they are called formula units)
6.02 x 1023 Na+ ions and
6.02 x 1023 Cl– ions
1 mole C
1 mole H2O
1 mole NaCl

24. Avogadro’s Number as Conversion Factor
6.02 x 1023 particles
1 mole or
1 mole
Note that a particle could be an atom OR a molecule!

25. Avogadro’s Number as Conversion Factor
How many moles of CO2 are in 25*1024 molecules of CO2 ???
1 mole CO2 = 6.02 * 1023 molecule
? mole CO2 = 25*1024 molecule
# mole CO2 = (25*1024 * 1)/ 6.02 * 1023
# mole CO2 = mole

26. Learning Check 1. Number of atoms in 0.500 mole of Al a) 500 Al atoms
b) x 1023 Al atoms
c) x 1023 Al atoms
2.Number of moles of S in 1.8 x 1024 S atoms
a) mole S atoms
b) mole S atoms
c) x 1048 mole S atoms

27. Learning Check
How many molecules are in 2.00 moles of H2O? 1.20x1024 molecules H2O
How many moles are in 8.30 X 1023 molecules of H2O? 1.38 moles H2O
How many molecules are in moles of H2? 3.01x1023 molecules H2
How many atoms are in 1.00 moles of He? 6.02x1023 atoms He
How many atoms are in 1.50 moles of Hg? 9.03x1023 atoms Hg
How many molecules are in 2.50 moles of O2? 1.51x1024 molecules O2
How many atoms are in moles of Rb? 1.51x1023 atoms Rb
How many moles are in 2.91 X 1022 atoms of He? moles Rb
How many moles are in 1.23 X 1050 molecules of H2? 2.04x1026 moles H2
How many moles are in 5.25 X 1025 atoms of Au? 87.2 moles Au
How many moles are in 7.14 X 1035 molecules of O2? 1.19x1012 moles O2
How many moles are in 8.11 X 1020 molecules of CH4? moles CH4