1. Stoichiometry Midterm
AP Chemistry
Ms. Diane Paskowski

2. Stoichiometry of Elements
Moles of an element
Molar mass – weighted average of the masses of the isotopes
Isotopes: same protons and electrons, different #s of neutrons so different masses
Abundance
Number of atoms of an element – use Avogadro’s number
Moles to grams to number of atoms and back

3. Stoichiometry of Compounds
Need a formula
Know nomenclature
Or % composition
Combustion analysis
Determine molar mass
Need molar masses of elements
Subscripts
Use molar mass to convert between mass and moles

4. Stoichiometry of Solutions
Solvent and solute
Water is universal solvent
Many kinds of solutions
Homogeneous mixtures
Occur in many states
Most common are liquid and gas
Electrolytic solutions – conduct electricity - have ions
Concentration
Molarity = moles/L
Others
Know how to make a solution of specific molarity
Volume, molar mass, desired molarity
Dilution
M1V1 = M2V2
Know Solubility Rules
Concentration of Ions in Solution
Does the compound dissociate?
How does it dissociate? Need ratio.
Common Ions
Mixtures of compounds within same solution – concentration of common ions is additive

5. Stoichiometry of Gases
Gas laws
Ideal Gas Law PV=nRT
Avogadro’s Law = V is directly related to moles. (constant P and T)
1 mol of any gas at STP has a volume of 22.4 L
Dalton’s Law of Partial Pressure
Ptot = P1 + P2 + P
Graham’s Law of Effusion
ra/rb = SqRT (Mb/Ma)

6. Stoichiometry of Reactions
Balanced chemical equation
Identify type of reaction (can be useful in problem solving)
Mole ratio = ratio of moles of unknown to moles of known
Subscripts identifying states
Write down all the given information and label it.
Calculate moles from
Volume and molarity (solutions)
Given mass (any)
Volume, pressure, temperature (gases)
% Yield
Actual/theoretical X 100
Multi-step yields. Multiply the % yield of each step to get the yield of the whole reaction. Or manipulate that calculation to get the % yield of an individual step
Titrations – Acid/Base or Oxidation-Reduction Reactions
Volume x Molarity = moles of known x mole ratio x 1/Vol = Molarity of unknown
Determine what you need to know, write it down and label it

7. Useful Constants for Stoichiometry
R = Universal Gas Constant or Rydberg’s Constant
L.atm/Mol K
8.314 J/mol K
NA = x 1023 anything = 1 mole
Useful conversion
T (K) = oC
1 atm = 760 mm Hg = 760 torr = 101,325 Pa
STP – Standard Temperature and Pressure
1 atm =P, 0 oC = T = 273 K
1 mol of any gas = 22.4 L
Density of water
1.0 g/ml at 4oC

8. Useful Equations for Stoichiometry
PV = nRT Graham’s Law va/vb = square root (Mb/Ma) Avogadro’s Law V/n = V’/n’ or 1 mol = 22.4 L (any gas at STP) Ptot = P1 + P Because P is proportional to moles Molar mass = g/ mol = g.mol-1 Ave mass element = S(fraction or percent abundance)x mass of each isotope (Mass x mole/g) x mole ratio = moles of unkown Moles of unknown x mole ratio = moles of known