A New Unit of Measurement

A Chemistry Exclusive!!

Dude!!!! Atoms are REALLY small!!!!!!

We can’t work with individual atoms or amu’s in the LAB– Because we can’t see things that small.

• We work with samples large enough for us to SEE and WEIGH on a balance using units of grams.

This creates a problem…..• A pile of atoms big enough for us to see

contains billions of atoms.– Billions of atoms are hard to keep track of in

calculations.

So chemists made up new unit..

What’s a MOLE?• It is a number of things

– Just like a “dozen”

– 1dozen eggs= 12 eggs– 1 mole eggs = 6.022 x 1023

POP QUIZ• 1 dozen Mg atoms =

12 Mg atoms

• 1 mole Mg atoms =6.022 x 1023 atoms

• 1 mole Mg(OH)2 =

6.022 x 1023 molecules

• 1 mole O2 =

6.022 x 1023 molecules

Everybody Dance Now!!!

“Moleshake”

My moleshake brings all the moles to the yard,

And they’re like, “Man, Chemistry’s hard”

Yeah right, yeah Chemistry’s hard,

I can teach you, and I won’t even charge

How Big is a Mole?• A mole of marbles would cover the entire

Earth’s surface 50 MILES DEEP!!!

How Big is a Mole?• If you spent a BILLION DOLLARS every

second, you couldn’t spend a mole of dollar bills in your lifetime!!!

Avogadro’s Number• 6.022 x 1023 is referred to as Avogadro’s

Number- In honor of Amedeo Avogadro, Italian chemist (1776-1865)

Ladies…..You know you want

me.

….and if Avogadro rings, ask him to leave his number…”

A “new” look at the periodic table…

• We already know the periodic table tells us the mass (in a.m.u.’s) of a single atom

– Ex. 1 Al atom has a mass of….

26.9 grams

Ex. 1 mole of Mg atoms has a mass of…

24.31 grams/mol

So MASS on the periodic table has two meanings…

• 1. When interpreted as grams/mole (g/mol) we call it “molar mass”

• 2. When interpreted as amu’s (g) we call it “atomic mass”

• The number doesn’t change, only the units.

This also applies to molecules

• The molecular mass of NH3 is…

– The mass of 1 individual NH3 molecule

14.01 amu

+ 1.01

+ 1.01

+ 1.01

17.04 amu (g) of NH3

Or… 17.04 g/mol of NH3 if you are interpreting for molar

mass.

Practice Problems

• 1. What is the atomic mass of Carbon?– 12.01 g or amu

• 2. What is the molar mass of Carbon?– 12.01 g/mol

• What is the molar mass of Mg3(PO4)2 ?– 262.87 g/mol

• How many moles of Oxygen are in 1 mole of Mg3(PO4)2 ?– 8 moles of Oxygen