Gram Formula Mass

Gram Formula Mass

Gram Formula Mass

This is a calculation of the mass of one mole of a compound. It is simply the sum of the masses of all the atoms in the substance. For example, a mole of LiCl, lithium chloride is 6.941+35.45. These are the masses on the periodic table for Li and Cl. So the answer is 42.391g. So if I weighed 42.391g exactly of LiCl, I would have 1 mole of the compound, or 6.02 x 10²³ molecules of LiCl.

Gram Formula Mass

Its another cool thing about the Periodic Table.

The atomic mass numbers can be used two ways!

We can use them to determine the mass of an atom in amu's (atomic mass units), and use it to determine the number of neutrons.

Or, we can change the unit to grams, and use it to determine the mass of one mole (6.02 x 10²³) atoms of any element.

Multiple Choice

What is a mole?

• We're in chemistry, so we are not talking about the little creature that digs holes , live underground and eats worms.

• A BIG number

• 6.02 x 10²³ of anything

• All of these

Calculations

So, one atom of Lithium has an average atomic mass of 6.941amu's. Look it up on the Periodic Table. The atomic number is 3, so the most common isotope of Lithium must have 4 neutrons. Remember this?

Calculations

Also, one mole of lithium atoms will have a mass of 6.941 grams.

Multiple Choice

You try it. What is the mass of one mole of Carbon atoms?

• 6 grams

• 6.02 x 10²³

• 12.01 grams

Calculations

Okay, so you got that. Now, what about magnesium bromide? MgBr₂? Well, you have one magnesium and 2 bromines. So The gram formula mass is 24.31 plus 2 times 79.90 because there are 2 bromines. So the gram formula mass is 184.11g.

Multiple Choice

You try it. One mole of NaF, sodium fluoride, will have a mass of

• 41.99g

• 20g

• 41.99 amu's

• 20amu's

Multiple Choice

What is the mass of one mole of sodium oxide Na₂O?

• about 62 grams

• 30g

• 30amu

• 60amu

What about Fe₂(SO₄)₃?

Well, how many of each atom does it have?

2Fe, 3S, 12O

So the atomic mass of Fe is 55.85, atomic mass of S is 32.07 and the atomic mass of O is 16.00

So 2 x 55.85 = 111.70

+3 x 32.07 = 96.21

+12 x 16.00 = 192.00

The gram formula mass = 399.91g. If I have 399.91g of iron(III) sulfate, I have 6.02 x 10²³ atoms, or one mole.

Multiple Choice

What is the gram formula mass for Mg(OH)₂?

• 41.3g

• 58.32g

• 65.6g