1. 5.3 – Molar Mass
Ms. Munir

2. Molar mass
The mass of one mole of a substance is called its molar mass (symbol M).
Molar mass of an element is equivalent to the atomic mass unit of that element.
E.g.
C – 12 atom has a.m.u. and 1 mole of C – 12 atoms weigh g/mol.

3. Finding Molar Mass of Compounds
Example – Find molar mass of CO2.
MCO2 = MC + 2MO
= g/mol + 2( g/mol)
= g/mol
One mol of CO2 = x 1023 = g/mol

4. Converting from moles to mass
Mass = # of moles x Molar mass
g = mol x g/mol
m = n x M
multiply by
Moles > Mass(g)
molar mass(g/mol) m M n

5. Example
A flask contains mol of CO2 gas. What mass of CO2 is in this flask?
Given: n = mol
m = ? g
Find M from periodic table { calculated above } = g/mol
m = mol x g/mol = 33.0 g
The mass of CO2 is 33.0g

6. Converting from Mass to moles
Divide by
M (g/mol)
Moles (mol)
<
Mass (g)
n = m / M
E.g. How many moles of acetic acid, CH4COO, are in a 23.6g sample?
Given: m = 23.6 g
n = ? mol

7. Solution
MCH4OO = 2MC + 4MH + 2MO
= 2(12.01) + 4(1.01) + 2(16.00)
= g/mol
n (mol CH3COOH)=23.6g/60.06g/mol
= 0.393mol
Therefore, there are 0.393mol of acetic acid in 23.6 g of acetic acid.

8. Converting between moles, mass and number of particles
Moles (mol)
Mass (g) NA
M(g/mol)

9. Example
What is the mass of 5.67 x 1024 molecules of cobalt (II) chloride, CoCl2?
Solution:
mass = ? g
N = 5.67 x1024 molecules
NA = x 1023 molecules/ mol
MCoCl2 = ( ) = g/mol

10. Solve by units

11. Example 2
Chlorine gas, Cl2, can react with iodine, I2, to form iodine chloride, ICl, how many molecules of ICl are contained in a 2.74x10-1 g sample?
m = 2.74 x 10-1 g
MICl = = g/mol
NA = x molecules/ mol
N = ? molecules

12. Solve by units

13. Practice
P 192 # 1 – 6 McGraw hill
P 277# 1 – 12 Nelson