To find the number of moles of a substance given the weight in grams:

1. Go to your Periodic table and find the weight of all of the elements in the compound (in this case K and then Cl).
2. The Periodic table tells you how many grams each element weights per mole of the element (just to note this is also the amount of atomic mass units (AMU) in one mole of the substance).
3. Now take the weight you have of the substance and multiply it by one mole/(atomic weights added together).
4. K weighs 39.0983 g/mole and Cl weighs 35.453 g/mole).

It should look like this:

                                               10 g KCl (1mol/74.1013gKCl) =

5. The grams will cancel and leave you with the number of moles.
6. Do not forget to find the number of significant figures and round at the end of all your calculations. NEVER round until you are done with all calculations.

Note: when doing all chemistry problems annotate each part of the problem with the identity of units you are using (like grams in this example) and with the substance (like KCl in this example).