10/07/03

When the mass of an atom in atomic miss units is expressed in an actual weight unit, the gram, we have one atomic mass in grams of the element.  This is known as the gram - atomic mass of the element.  For example, the atomic mass of one atom of hydrogen is one atomic mass unit, but the mass of one mole of atoms of hydrogen is 1g .

Example one:

How many moles of potassium or there and 78g of the elements? 

Step one: look up the atomic mass of potassium and  on the periodic table.  It is 39g. 

Step two: divide the number of a grams of the potassium into the total mass of the element.

Answer: 78g / 39g = 2 moles

Example two:

How many grams of carbon are there in 3.5 moles of the element? 

Step one: get carbon's atomic mass from the periodic table.  It is 12g / mole . 

Step two:  Multiply the mass per mole by the number of of moles of the element.

Answer: (12g / mole)(3.5 moles) = 42g of C

Gram molecular mass and the mole

The molecular mass of any compound can be determined by finding the sum of the gram atomic masses of the atoms that make up the molecule.  When the sum of the atomic masses is expressed in, it is called the gram molecular mass of that substance.  for example, the molecular mass of water it is 18 atomic mass units.  When this number is expressed in grams it is the gram molecular mass of water.

Example three:  

What is the gram molecular  mass of sulfuric acid, H₂SO₄?

Step one:

Write the symbols of elements making up the molecule in a column:

H

S

O

Step two:

Indicate the number of atoms of each element in the compound:

H       2

S       1

O       4

Step three:

Obtain the gram atomic mass of each element from the periodic table.

H       2        1 g

S       1        32 g

O       4       16 g

Step four:

Multiply the number of the atoms of the elements by their respective masses:

H       2    X    1 g     =     2 g

S       1    X    32 g    =    32 g

O      4    X    16 g    =    64 g

Step five:

Find a sum of all the atomic masses:

2 g + 32 g  + 64 g = 98g H₂SO₄