How to Count AtomsWhat unit is used to count donuts?Would that unit be appropriate for counting the

# of people in Jacksonville?Would it be appropriate for counting # of atoms

in a drop of water?We need an appropriate unit for counting things

that are too small to see!!

What is mole? Mole Ratio A Ratio of the Amounts of Reactants and Products using the Coefficients of a chemical reaction.

Mole is important in Stoichiometry part of chemistry that studies amounts of substances that are involved in reaction.

Ex. 1 Cu (s) + 2 AgNO3 (aq) → 2 Ag (s) + Cu(NO3)2(aq)

The Mole Concept:

a.mole- a counting unit, like a dozen, but MUCH larger, a mole is always equal to Avogadro’s numberb. Avogadro’s # = 6.022 x 1023 = 1 mole

b/c atoms are so small, their counting unit is HUGE!!, a mole can be used to count anything at the atomic or molecular level

**Avogadro’s # is a conversion factor between moles and particles**

c. Molar mass- numerically equivalent to amu but in units of #g/mol, also a conversion factor between grams & moles 

Molar Mass is a mass of one (1) mole of a substance or any element or compound.

Mole is a unit of measurement used in chemistry to express the number of particles of a substance.

1 mole = 6.02 x1023 particles

Examples: What is the molar mass of Carbon?

What is the molar mass of Alum inum?

Compounds: H2O , H2SO4 , CaCl2 . 2H2O

*Mole as a Conversion FactorEx.1 How many particles are in 4.2 moles?4.2 mol x 6.022x1023part.=

1 mole2.5x1024 particles

EX. 2 How many moles are 4.3 x1020 atoms?4.3x1020 atoms x 1mole =

6.022x1023atoms 7.1x10-4 moles

Mole to Mole Conversion1. How many moles of H2O will produced by

4.8 moles of O2 ?

2C4H10(g) + 13O2 (g) → 8CO2(g) + 10 H2O(g)

If there are 5.6 moles of Oxygen, how many moles of CO2 can be produced?

C2H2 (g) + O2 → CO2(g) + H2O(g)

Molar mass and MolesProblem: You have 16.5 g of oxalic acid, H2C2O4

a.What amount(moles) is represented by 16.5 g of oxalic acid?

b. How many molecules of oxalic acid are in 16.5 g?

c. How many atoms of carbon are in 16.5 g of oxalic acid?

Relating Moles to Mass Molar Mass of a substance…1. Is the mass in g of exactly 1 mole

(6.022x1023 particles) of that substance2. Has the units some # g per 1 mole Ex. for C 12.011g = 1mole Means that 1 mole of C weighs 12.011 g

Means that 6.022x1023 atoms of C weigh 12.011 g

3. Is numerically equal to the average atomic mass but in more useful units, g/mol

4. Is used as a conversion factor

Examples:• Molar Mass: (Formula Mass)CuSO4

H2O

MgCl2

Na3PO4

*Molar Mass as a Conversion FactorEx. 1 How much will 7.51 moles of Al weigh?7.51 mol Al x 26.982 g Al = 1 mol Al

203 g AlEx. 2 How many moles are in a 15.2 kg sample of

lithium?15.2 kg x 1000g x 1mole Li = 1 kg 6.94 g Li

2190 mol Li

How to calculate % Composition?*find molar mass*divide mass of the part you are looking

for by molar mass and multiply by 100%

*Ex. %O in H2O?Molar mass of H2O is 18 g/molMass of O in H2O is 16 g16 g/18 g * 100% = 88.8%, so water

is 88.8% O 

More examples… Percent Composition CO2

C2H5OH (ethanol)

 NaHSO4

C2H6

• It is a way to represent a formula including waters of hydration, which are water molecules incorporated into the crystal structure of the salt.

*To find the molar mass of a hydrate ADD (not multiply) all the waters of hydration.

Ex. CuSO4●5H2O = 5 waters of hydration so ADD 5 x 18 (molar mass of water) to molar mass of CuSO4

 

Practice A compound with a mass of 48.72 g

is found to contain 32.69 g of zinc and 16.03 g of sulfur. What is the percentage composition of the compound?