Dr. Walt Volland revised October 12, 2009

  

Molar mass for compounds

The formulas for compounds are familiar to you. You know the formula for hydrogen peroxide is H2O2. It should be reasonable that the weight of a formula unit can be calculated by adding up the weights for the atoms in the formula.The weights are read from the periodic table.

The formula weight for H2O2 = weight from hydrogen + weight from oxygen

The formula weight for H2O2 = 2 H atoms x 1.008 amu + 2 O atom x 16.00 amu = 34.016 amu

The molar mass for H2O2 = 34.016 grams H2O2
 

Example 1

How many grams of hydrogen are needed to give 3. moles of hydrogen?
1. Calculate the molar mass for hydrogen. Look up the atomic weight/mass in the periodic tableThe molar mass for hydrogen is 1 mole H2 = 2.016 grams H2

2. determine the mass needed to provide 3. moles of hydrogen.

1 mole H2 = 2.016 grams H2;

2 mole H2 = 4.032 grams H2

3 mole H2 = 6.048 grams H2

The practical way is to multiply the molar mass by the number of moles.

This converts mole to grams

(3 mole H2 )(2.016 grams H2/ 1 mole H2 ) = 6.048 grams H2

Here the label moles H2 will cancel out. return to top of page

Example

How many moles of water are in a liter of water? Assume 1 liter = 1 kilogram water

1. Calculate the formula weight(mass) for water, H2O.

Look up the atomic weights in the periodic table for H and O.
The atomic weight for hydrogen is 1.008 amu
The atomic weight for oxygen is 16.00 amu

2. Add up the masses from all the atoms in the formula

The formula weight for water is 1.008 amu + 1.008 amu + 16.00 amu = 18.016 amu

3. Determine the molar mass for water. Molar mass is a mass in grams that is numerically the same as the formula weight.

1 mole H2O = 18.016 grams H2O = 18. grams H2O ( to the nearest gram )

4. Convert 1000 grams of water to moles. The "conversion factor" is the molar mass.

(1000 grams H2O )(1 mole H2O/ 18. grams H2O) = 55.55 moles H2O

This is 56 moles H2O (rounded to 2 sf) return to top of page

Example

How many moles of sulfur dioxide, SO2 (g), are in 2000 grams of the gas?

 

1. Look up the atomic weights in the periodic table for S and O.

The atomic weight for sulfur is 32.07 amu
The atomic weight for oxygen is 16.00 amu

2.Calculate the formula weight for SO2 . Add up the masses from all the atoms in the formula

The formula weight for sulfur dioxide is

32.07 amu S + 2 x (16.00 amu O) = 64.07 amu SO2

3. Determine the molar mass. Molar mass is a mass in grams that is numerically the same as the formula weight.

1 mole SO2 = 64.07 grams SO2 = 64. grams SO2 (to the nearest gram )

4. Convert 2000 grams of SO2 to moles. The "conversion factor" is the molar mass.

(2000 grams SO2 )(1 mole SO2 /64. grams SO2 ) = 31.25 moles SO2 = 31 moles SO2 (rounded to 2 sf)

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Dr. Walt Volland, All rights reserved, 2005-2009
last modified October 12, 2009