Calculating weight, particles, and moles

The mole is the bridge between the microscopic and macroscopic world:  6.022 × 10²³ particles ↔ 1 mole ↔ atomic/formula weight in grams

 If you have any one of the three things — particles, moles, or grams — then you can calculate the other two. For example, the weight of a water molecule is 18.015 amu.

Because a mole is the formula (or molecular) weight in grams of a compound, you can now say that the weight of a mole of water is 18.015 grams. You can also say that 18.015 grams of water contains 6.022 × 10²³ H₂O molecules, or a mole of water.

And the mole of water is composed of two moles of hydrogen and one mole of oxygen.

Suppose you want to know how many water molecules are in 5.50 moles of water. You can set up a problem like this:

5.50 mol × 6.022 × 10²³ molecules/mol = 3.31 × 10²⁴ molecules.  Or suppose that you want to know how many moles are in 25.0 grams of water. You can set up the problem like this:

You can even go from grams to particles by going through the mole. For example, how many molecules are in 100.0 grams of carbon dioxide? The first thing you have to do is determine the molecular weight of CO₂. Look at the periodic table to find that one carbon atom equals 12.011 amu and one oxygen atom weighs 15.000 amu. Now figure the molecular weight, like this:

[(1 × 12.011 g/mol) + (2 × 15.999 g/mol)] = 44.01 g/mol for CO₂

Now you can work the problem: And going from particles to moles to grams is just as easy.