Molar Mass Mass of Atoms

 Molar Mass
-The mass ( in grams ) of 1 mole of a substance is called the molar mass
- It can be determined from the atomic mass on the periodic table
- Measured in g/mol

Molar Mass of Compounds

- To determine the molar mass of a compound add the mass of all atoms together

Example

Element                  Molar Mass              *Significant Digits

H2O                   2 + 16 = 18.0g/mol  
NO2                  14 + 32 = 46.0g/mol
NaCl                  23 + 35.5 = 58.5g/mol  
FeO                   55.8 + 16 = 71.8g/mol
NaNO3             23 + 14 + 48 = 85.0g/mol

 Converting between moles and mass
- To convert between moles and mass we use molar mass as the conversion factor
- Be sure to cancel the appropriate units

Examples

How may grams is there in 1.5 mol of O2?


1.5mol O2 x 32.0g   = 48g
                  1 mol O2


Example

A sample of HCl contains .54 mol. How many grams of HCl is this?

.54mol HCl x 36.5g     = 20g HCl
                    1 mol HCl


Example

A compound is made of phosphorus and chlorine. It is found to contain 0.200 mol and has a mass of 27.5g
- Determine the molar mass of the compound
- Suggest a possible formula

0.200mol x 27.5g  =  137.5g/mol            Formula = PCl3
                        1 mol