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Module 3.09
Literacy
Molar Mass of
Compounds
**Please make sure you have a periodic
table, calculator and something to write with

Standards and Objectives
Objectives:
-Calculate the molar mass of a given
compound
-Determine the empirical formulas from
given data
Standard:
SC.912.P.8.7—Interpret formula
representations of molecules and compounds
in terms of composition and structure.
Molar Mass
What do we know about molar mass?
What do we Want to know?
Molar Mass
What do we know about molar mass?
-molar mass = atomic mass of an element
What do we Want to know?
-Calculate molar mass for a compound and
use it in conversions
Review: Formulas and Subscripts
Determine the number of each atom in the
compound below:
1.
2.
3.
List the elements in the compound
Determine the number of atoms of each element using subscripts
For polyatomic ions (in parentheses) subscripts on the outside
multiply each element of the polyatomic ion.
Ca(NO3)2
Ca:
N:
O:
This will be used to calculate molar mass for the compound.
Molar mass
Molar mass of an element = atomic mass for
that element from the periodic table.
Molar mass of a compound or molecule = the
sum of atomic mass of each element x
subscript (# of moles)
Calculating Molar mass of a Compound
1. List the number of atoms of each element.
2. Multiply the # of each element by it’s atomic mass to
determine the molar mass for each element.
3. Determine the molar mass of the compound by taking the
sum of the molar mass of each element.
Ex. Determine the molar mass of the compound
below. Ca(NO3)2
Ca: 1 x
N: 2 x
O: 6 x
Calculating Molar mass of a Compound
1. List the number of atoms of each element.
2. Multiply the # of each element by it’s atomic mass to
determine the molar mass for each element.
3. Determine the molar mass of the compound by taking the
sum of the molar mass of each element.
Ex. Determine the molar mass of the compound
below. Ca(NO3)2
Ca: 1 x 40.08 g =40.08 g
N: 2 x 14.01 g = 28.02 g
O: 6 x 15.999 g = 95.99g.
(40.08 + 28.02 + 95.99) = 164.094 g/mol
Molar Mass Practice
Determine the Molar mass for the following
compounds:
H3PO4
Al(NO3)3
P2O5
Molar Mass Practice
Determine the Molar mass for the following
compounds:
H3PO4
(3 × 1.0079 g/mol H) + (1 × 30.973 g/mol P) + (4 × 15.999 g/mol O)
= 97.993 g/mol H3PO4
Al(NO3)3
(1 × 26.981 g/mol Al) + (3 × 14.006 g/mol N) + (9 × 15.999 g/mol O)
= 212. 99 g/mol Al(NO3)3
P2O5
(2 × 30.973 g/mol P) + (5 × 15.999 g/mol O) = 141.94 g/mol P2O5
Using Molar Mass Calculations
You can use the molar mass of compounds in dimensional analysis
calculations just as you used the molar mass of individual atoms. Use the
the periodic table to look up the molar mass of each element.
Convert grams to moles
Grams A given
1 mol A
= _____ mol A
molar mass g of A
Convert moles to grams
Moles A given
molar mass g of A
1 mol A
= ______ g A
Using Molar Mass Calculations
You can use the molar mass of compounds in dimensional analysis
calculations just as you used the molar mass of individual atoms. Use the
the periodic table to look up the molar mass of each element.
Ex. 1 What is the mass of 3.85 moles of oxygen gas (O2)?
Given:
What formula will we need to use?
Want:
Using Molar Mass Calculations
Ex. 1 What is the mass of 3.85 moles of oxygen gas (O2)?
Given: 3.85 mol O2
Want: mass of O2
Which conversion will we use?
convert mol to g
Determine molar mass of O2: 2x 15.999 g = 31.998 g/mol
3.85 mol O2
31.998 g O2
= 123 g O2
1 mol O2
Using Molar Mass Calculations
Ex. 2 How many moles of barium nitrate (Ba(NO3)2) are in 305.5 g
Ba(NO3)2?
Given:
Which conversion will we use?
Want:
Using Molar Mass Calculations
Ex. 2 How many moles of barium nitrate (Ba(NO3)2) are in 305.5 g
Ba(NO3)2?
Given: 305.5 g Ba(NO3)2
Want: moles Ba(NO3)2
Which conversion will we use?
convert g to mol
Determine molar mass of Ba(NO3)2 :
(1 x 137.32 g) + (2 x 14.01 g) + (6 x 15.999) = 261.334 g/mol Ba(NO3)2
305.5 g Ba(NO3)2
1 mol Ba(NO3)2
=
261.334 g Ba(NO3)2
1.169 mol Ba(NO3)2
Empirical Formulas
Determining Empirical Formula
Empirical Formula Example
Given Values
Molar Mass
What do we know about molar mass?
-molar mass = atomic mass of an element
What do we Want to know?
-Calculate molar mass for a compound and
use it in conversions
What did we learn?
-How to calculate molar mass using
subscripts and atomic mass
How to determine empirical formulas from
data.

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