How can atoms be counted? When we want to know how many atoms of a substance are in a sample of the substance that we can see, counting the atoms individually is not practical. First of all, atoms are so small that we cannot see them individually with our eyes. Second, we would be counting for a very, very, very long time.

Here is an example for you to think about. The "lead" of a pencil is made of pure carbon (graphite). Think about the size of the dot on a "i" that you would write on your paper with a pencil. Not very big right?. The number of atoms of carbon in that dot is roughly equal to the population of the world - about 6 billion! It would take a very long time to count that high. And that is just for the number of atoms in the dot on an "i" on your paper!

So then what is the method to count? Chemist use a unit for counting the number of atoms in a sample of matter. This unit is called the MOLE. One mole is equal to x atoms of any element. Using the term "mole" is similar to using the word "dozen". We all know that a dozen means 12. Well, a mole means x

How can we count atoms if we can’t see them? In order to count atoms we will use “Molar Mass” Molar mass is the average mass of all of an elements isotopes measured in grams. For example, the average atomic mass of carbon on the Periodic Table is Therefore, grams of carbon is equal to one mole of carbon.

Molar Mass and “The Table” To get the molar mass of an element on the periodic table its simple….. Just look at is mass For a compound we do the same thing except we add up all of the different elements weights Examples: Water (H 2 O) 2 H’s and 1 O that’s (1.008 x 2) + 16 = 18 g/mol Na 2 SO 4 = (22.99 x 2) + (32.07) + (16 x 4) =142g/mol

Molar Mass and Avogadro’s # Since we just learned a moment ago that one mole of carbon is equal to x atoms of carbon therefore we can say… x atoms of carbon have a mass of grams. or grams is the mass of x atoms of carbon So what chemist are saying is…. 1 mole of any element = x atoms or molecules or particles of that element = molar mass of element or cpd

How does this work Mathmatically? Simple: 1. Just like before write down your given and unknown for a problem. 2. Identify if you given is in moles, atoms or grams. 3.Use dimensional analysis (factor label) to convert your given to your unknown unit.

Here is a chart that might help You will not be able to use this on any assessment. It is only to help you get started. DIVIDE BY MOLAR MASS (MM) DIVIDE BY X > MULTIPLY BY MOLAR MASS (MM) MULTIPLY BY X 10 23

Let’s try some!!! Determine the number of moles in 1.25 x atoms of H 2 SO 4 Given= 1.25 x atoms of H 2 SO 4 Unknown number of moles of H 2 SO 4 Conversion: Atoms to moles. No mass needed 1.25 x atoms H 2 SO 4 1 mole H 2 SO x atoms H 2 SO 4

How do I type this on my Calculator? 1.25 x atoms H 2 SO 4 1 mole H 2 SO x atoms H 2 SO 4E In your calculator you will type: nd EE 23 multiplied by 1 (1.25 E 23 x 1) Divided by nd EE 23 (6.022 E 23) Answer is=.208 Moles H 2 SO 4

A. Percentage Composition the percentage by mass of each element in a compound

 100 = A. Percentage Composition %Cu = g Cu g Cu 2 S  100 = %S = g S g Cu 2 S % Cu 20.15% S Find the % composition of Cu 2 S.

%Fe = 28 g 36 g  100 = 78% Fe %O = 8.0 g 36 g  100 = 22% O Find the percentage composition of a sample that is 28 g Fe and 8.0 g O. A. Percentage Composition

How many grams of copper are in a 38.0-gram sample of Cu 2 S? (38.0 g Cu 2 S)( ) = 30.3 g Cu Cu 2 S is % Cu (from before) A. Percentage Composition

 100 = %H 2 O = g g 24.51% H 2 O Find the mass percentage of water in calcium chloride dihydrate, CaCl 2 2H 2 O? A. Percentage Composition