example:

98.93 % 12^c & 1.07% 13^C  masses are 12 amu, and 13.00335.

(0.9893)(12amu) + (0.0107)(13.00335amu) = 12.01amu

a.k.a Atomic Weight

Significant Figures:

Addition and Subtraction

the result has the ____ number of decimal places as the measurment with the ______ decimal places.

How to calculate mass number

^{ex: 12}C

How to calculate Atomic number

_{ex. 6}C

 It states that the masses of one element which combine with a fixed mass of the second element are in a ratio of whole numbers

postulate 3

Rule #1 of significant figures

Zeros between nonzero digits

Always important

ex. 1005 4 sig figs

ex. 1.03 3 sig figs

Rule # 2 signifigant figures

Zeros at the beginning of a number

NEVER significant

(merley indicate the position of the decimal point)

ex. 0.02 one significant figure

Rule #3 of significant figures

Zeros at the end of a number

Are significant if the number contains a decimal point

ex. 0.0200 3 significant figures

ex. 3.0 2 significant figures     

Significant figures:

In any measurment that is poroperly reported, all nonzero digits are ____

The sum of the atomic weights of each atom in it's chemical formula.

ex.12 C atoms = 12(12.0amu) = 144.0amu       

22 H atoms = 22(1.0) = 22.0amu            

11 O atoms = 11(16.0 amu) = 176.0 amu

Total: 342.0 amu (Formula Weight)

Temperatue, melting point, and density

(do not depend on quantity)

(depend on the quantity of the sample)

mass and volume

If the chemical formula is that of that of a molecule, then the formula weight is also called the moecular weight.

ex. glucose C₆H₁₂O₆

_{MW :6(12.0amu)+12(1.0amu)+6(16.0amu)=180amu}

Percent Composition from Formulas

%element =

 ^{(Number of atoms of that element) (atomic weight of element)}

Formula weight of compound

x 100%

Estimating Numbers of Atoms

Ex. Without using a calculator, arrange the following samples in order of increaasing numbers of carbon atoms:12g ¹²C, 1 mol C₂H₂, 9 x 10²³ molecules of CO_2.

1. Analyze- We are given amounts of different substances expressed in grams, moles, and a number of molecules and asked to arrange the samples in order of increasing numbers of C atoms.

2. Plan- To determine the number of C atoms in each sample, we must convert g ¹²C,1 mol C₂H₃ and 9 x 10²³ molecules CO₂ all to numbers of C atoms. To make these conversions, we use the definition of mole and Avogadro's number.

3. Solve- A mole is defined as the amount of matter that contains as many units of the matter as there are C atoms in exactly 12 g of ¹²C. Thus, 12g of ¹²C contains 1 mol of C atoms (that is 6.02 x 10²³ atoms). One mol of C₂H₂ molecule, this sample contains 12 x 10²³ C atoms. Because each CO₂ molecule contains one C atom, the sample of CO₂ contains one C atom, the CO2 contains