1. Chemical Formulas & Mass
Foothill Chemistry
Chemical Formulas & Mass

2. Chemical Formulas Review
Chemical Formula - Indicates the relative numbers of atoms of each kind in a chemical compound by using atomic symbols and numerical subscripts.
Molecular Formula – Chemical formula for a covalent compound
Formula Unit – Chemical formula for an ionic compound

3. Chemical Formulas Review
Lewis Structure – Chemical formula that shows bonding between elements and the unpaired electrons
Structural formula – Like a Lewis structure, but no unpaired electrons (and sometimes no hydrogen) – But really a Lewis structure is a type of structural formula
Empirical Formula – Shows the composition of a compound in terms of the simplest ratio of atoms (subscripts are a reduced fraction)

4. Oxidation Numbers / Oxidation States
Indicate the general distribution of electrons among the bonded atoms in a molecular compound or polyatomic ion
Oxidation numbers are useful in naming compounds, writing formulas, and balancing chemical equations.

5. Oxidation Number Rules
Atoms of a pure elements have an oxidation number of zero.
The more electronegative element in a binary compound is assigned as the anion, the less electronegative element is assigned as the cation.
Fluorine always has an oxidation number of negative 1 (-1) in compounds.

6. Oxidation Number Rules
Oxygen almost always has an oxidation number of -2, except in peroxides when it has an oxidation number of -1. With fluorine it will have a +2 charge (OF2)
Hydrogen has a +1 oxidation number with compounds containing elements that are more electronegative than it is. It has an oxidation number of -1 in combination with metals.

7. Oxidation Number Rules
The algebraic sum of all oxidation numbers in a neutral compound is zero.
The algebraic sum of all oxidation numbers in an ionic compound is the charge of the ion.
In ionic compounds, the ions have oxidation numbers equal to the charge of the ion.

8. Oxidation Number Practice
UF6
H2SO4
ClO3-

9. Formula Mass
The sum of the average atomic masses of all atoms represented in its formula
Formula mass is a general term used for molecules, formula units, and ions.
Molecular mass is used for molecules and polyatomic ions.
Units are in amu

10. Molar Mass Mass of one mole of a substance.
Numerically equivalent to the formula mass.
Units are g/mol
Oh no! Moles are back!

11. Formula Mass / Molar Mass Practice
H2SO4
Ca(NO3)2
PO43-
MgCl2
Al2S3

12. Percent Composition Mass of element in sample / mass of sample x 100%
Mass of element in 1 mol / m0lar mass x 100%
The percentage by mass of each element in a compound is known as the Percent Composition of the compound.

13. Determining Empirical Formula From Percent Composition
Find the number of moles of each element in the compound.
Divide each number by the smallest number of moles
The smallest whole number ratio is the empirical formula

14. Empirical Formula Practice (Together)
32.38% sodium, 22.65% sulfur, 44.99% oxygen – What is empirical formula?
Sample Mass g; Phosphorus mass 4.433g; only contains phosphorus and oxygen – What is empirical formula?

15. In Class Assignment – Due Today
Determine the Empirical Formula for the Following:
75% Carbon, 25% Hydrogen
52.7% Potassium , 43% Chlorine
22.1% Aluminum, 25.4% Phosphorus, 52.5% Oxygen
13% Magnesium, 87% Bromine
32.4% Sodium, 22.5% Sulfur, 45.1% Oxygen