1. Bell Ringer
Determine the molecular weight of sugar (C12H22O11) and the formula weight of Potassium dichromate.
Sugar: 342
K2Cr2O7: 294

2. Calculations with Chemical Formulas

3. Review Formula Weight (FW)
Sum of the atomic weights for the atoms in a chemical formula
So, the formula weight of calcium chloride, CaCl2, would be
Ca: 1(40.1 amu)
+ Cl: 2(35.5 amu)
111.1 amu
These are generally reported for ionic compounds

4. Review Molecular Weight (MW)
Sum of the atomic weights of the atoms in a molecule
For the molecule ethane, C2H6, the molecular weight would be
C: 2(12.0 amu)
+ H: 6(1.0 amu)
30.0 amu

5. Percent Composition
Calculating the percentage composition of any element in a substance is straightforward if the chemical formula is known.
The calculation depends on:
The formula weight of the substance
The atomic weight of the element of interest
The number of atoms of that element in the chemical formula:
% element =
(number of atoms)(atomic weight)
(FW of the compound)
x 100

6. Percent Composition
The percentage of carbon and H in ethane, C2H6, is…
%C =
(2)(12.0 amu)
(30.0 amu)
24.0 amu
30.0 amu =
x 100
= 80.0%
%H =
(6)(1.00 amu)
(30.0 amu)
6 amu
30.0 amu =
x 100
= 20.0%

7. Calculate the percentage of carbon, hydrogen, and oxygen (by mass) in C12H22O11.

8. Counting particles
In chemistry the counting unit for numbers of atoms, ions, or molecules in a laboratory-size sample is the mole, abbreviated mol.*
One mole is the amount of matter that contains as many objects (atoms, molecules, ions, etc.) as the number of atoms in exactly 12 g of isotopically pure C-12
*The term mole comes from the Latin word moles, meaning “a mass.” The term molecule is the diminutive of this word and means “a small mass.”

9. Avogadro’s Number 6.02 x 1023 atoms 1 mole of 12C has a mass of 12 g
From experiments, scientists have determined this number to be *1023, which we will usually round to 6.02 * 1023.
Scientists call this value Avogadro’s number, NA in honor of the Italian scientist Amedeo Avogadro (1776–1856).
6.02 x 1023 atoms
1 mole of 12C has a mass of 12 g

10. SAMPLE EXERCISE 3.7 Estimating Numbers of Atoms
Without using a calculator, arrange these samples in order of decreasing numbers of carbon atoms: 12 g 12C, 1 mol C2H2, 9 * 1023 molecules of CO2.
Solve
12 g of C-12 contains 1 mol of C atoms= 6.02 * 1023 C atoms
One mol of C2H2 contains 6.02 * 1023 C2H2 molecules. Because there are two C atoms in each molecule, this sample contains 12 * 1023 C atoms.
Because each CO2 molecule contains one C atom, the CO2 sample contains 9 * 1023 C atoms.
Hence, the order is: 1 mol C2H2(12 * 1023 C atoms )> 9 * 1023 molecules of CO2(9 * 1023 C atoms) > 12 g 12C (6.02 * 1023 C atoms)