Module 3.09Literacy

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 dataStandard: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. List the elements in the compound2. Determine the number of atoms of each element using subscripts3. 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 = ______ g A

1 mol 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 Which conversion will we use? Want: mass of O2 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 Which conversion will we use? Want: moles Ba(NO3)2 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

= 1.169 mol Ba(NO3)2

261.334 g 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 elementWhat do we Want to know?-Calculate molar mass for a compound and use it in conversionsWhat did we learn?-How to calculate molar mass using subscripts and atomic massHow to determine empirical formulas from data.