How can atoms be counted?

•When we want to know how many atoms of asubstance are in a sample of the substancethat we can see, counting the atomsindividually is not practical. First of all, atomsare so small that we cannot see themindividually with our eyes. Second, we wouldbe 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 apencil. Not very big right?. The number of atomsof carbon in that dot is roughly equal to thepopulation of the world - about 6 billion! Itwould take a very long time to count thathigh. And that is just for the number of atoms inthe dot on an "i" on your paper!

So then what is the method to count?

Chemist use a unit for counting the number ofatoms in a sample of matter. This unit is calledthe MOLE.

•One mole is equal to 6.022 x 1023 atoms of anyelement.

•Using the term "mole" is similar to using theword "dozen". We all know that a dozen means12.

•Well, a mole means 6.022 x 1023.

How can we count atoms if we can’tsee them?

•In order to count atoms we will use “MolarMass”

•Molar mass is the average mass of all of anelements isotopes measured in grams.

•For example, the average atomic mass ofcarbon on the Periodic Table is 12.011.

•Therefore, 12.011 grams of carbon is equal toone mole of carbon.

Molar Mass and “The Table”

•To get the molar mass of an element on theperiodic table its simple….. Just look at is mass

•For a compound we do the same thing exceptwe add up all of the different elements weights

Examples:

Water (H2O) 2 H’s and 1 O that’s (1.008 x 2) + 16= 18 g/mol

Na2SO4 = (22.99 x 2) + (32.07) + (16 x 4)=142g/mol

Molar Mass and Avogadro’s #

•Since we just learned a moment ago that onemole of carbon is equal to 6.022 x 1023 atomsof carbon therefore we can say…..

6.022 x 1023 atoms of carbon have a mass of 12.011 grams.

12.011 grams is the mass of 6.022 x 1023 atoms of carbon

So what chemist are saying is….

1 mole of any element = 6.022 x 1023 atoms or molecules orparticles of that element = molar mass of element or cpd

How does this work Mathmatically?

•Simple:

1. Just like before write down your given andunknown for a problem.

2. Identify if you given is in moles, atoms orgrams.

3.Use dimensional analysis (factor label) toconvert 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 6.022 X 1023 ------------------------------------> <-----------------------------------GRAMS MOLES ATOMS OR MOLECULES <------------------------------------ ------------------------------------> MULTIPLY BY MOLAR MASS (MM) MULTIPLY BY 6.022 X 1023