Chapter 3 - Stoichiometry

• Chemical equations
• 2H2 + O2 2H2O
• H2 and O2 are reactants.
• H2O is a product.
• indicates where reactants end and products begin
• similar to equal sign!
• Numbers in front indicate atoms/molecules
• stoichiometric coefficients (SC)
• must be integers
• 1 is assumed for O2. (similar to algebra.)
• Balancing - same # of each type of atom on each side of rxn.
• CANNOT change subscripts
• Change only SC
• CH4 + O2 CO2 + H2O

• Patterns of Chemical Reactivity
• Periodic table to help predict rxn.
• 2K + 2H2O 2KOH + H2
• use to predict rxn of Na, Rb, etc.
• C/H/O - combustion
• form CO2 + H2O
• Combination - form 1 product
• Decomposition - from 1 reactant

• Atomic/Molecular weight
• Atomic weight (AW) - weighted average of all isotopes observed in nature.
• a.m.u. scale - based on mass of 12C = 12 a.m.u.
• not particularly useful.
• Mole
• # of objects (6.0221367 x 1023) - Avogadro's number
• # of atoms of 12C in exactly 0.012 kg (12 g)
• Allows use of reported atomic weights in units of grams/mole (g/mol)
• EXTREMELY USEFUL!!!
• Molecular weight (MW).
• Formula weight - better term
• also called molar mass.
• sum of AW of all atoms in molecule/formula.
• most useful units are g/mol.
• Must master skill of calculating MW from AWs.
• MW is JUST ANOTHER CONVERSION FACTOR!
• use to convert from mass to moles of substance
• MW specific to a substance.

• Empirical formula calculation
• When given % by mass information.
• Assume 100g - then % = mass in g
• Convert mass to moles (CAR)
• divide all values by the smallest # of moles
• find whole number ratio.
• If value x.5, multiply by 2
• If value x.33, x.67, multiply by 3
• When given actual mass
• Convert mass to moles (CAR)
• divide all values by the smallest # of moles
• find whole number ratio.
• If value x.5, multiply by 2
• If value x.33, x.67, multiply by 3
• Molecular formula
• Divide MW by EW
• If given MW and % comp
• Assume mass = MW
• MASS element = MW * % element / 100
• calculate like empirical formula
• moles should be very close to integers - these are the integers for the MOLECULAR FORMULA!
• Combustion analysis - read

• Quantitative information
• Master calculation gAmolAmolBgB

• Limiting reactant (LR) - reactant that you run out of first!
• gasoline/air
• sandwich
• To determine
• calculate moles of each reactant
• divide by SC
• choose smallest value - this is LR
• Theoretical yield (TY) - how much you should get
• Actual yield (AY) - how much you do get
• less than TY - side rxns, physical loss, incomplete rxn, etc...
• % yield = AY/TY

Learning Goals

1. Balance chemical equations.

2. Predict the products of a chemical reaction having seen a suitable analogy.

3. Predict the products of the combustion reactions of hydrocarbons and simple compounds containing C, H, and O atoms.

4. Interconvert number of moles, mass in grams, and number of atoms, molecules, or ions. [Instructor's note - the conversions in bold face are the ones I consider to be critical]

5. Calculate the empirical formula for a compound having been given the amounts or percentages of elements present in a compound.

6. Calculate the molecular formula, having been given the empirical formula and the molecular weight.

7. Calculate the mass of a particular substance consumed or produced in a chemical reaction.

8. Determine the limiting reactant in a problem.

9. Calculate the theoretical yield and/or the percent yield for a substance.