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?
