Stoichiometry Midterm AP Chemistry Ms. Diane Paskowski

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

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

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

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 + P3 . . . . . Graham’s Law of Effusion ra/rb = SqRT (Mb/Ma)

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

Useful Constants for Stoichiometry R = Universal Gas Constant or Rydberg’s Constant 0.08206 L.atm/Mol K 8.314 J/mol K NA = 6.022 x 1023 anything = 1 mole Useful conversion T (K) = oC + 273.15 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

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 + P2 + . . . 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