Chemistry 1364

General Chemistry I

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 reaction.
• CANNOT change subscripts
• Change only SC

• Problems: 1-7 odd

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

• Problems: 9-13 odd

• 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.
• Masses very small
• Difficult to measure without sophisticated instruments (which usually destroy the sample!)
• Rare for an atom to have an atomic weight equal to the 'average' atomic weight.
• Mole
• # of objects (6.0221367 x 1023)
• # 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 the molecule or formula.
• most useful units are g/mol.
• NOTE: You MUST master skill of calculating a molecular weight from the atomic weights!

• MW is JUST ANOTHER CONVERSION FACTOR!
• use to convert from mass to moles of substance
• MW specific to a substance.

• Problems: 15 41 odd

• 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 Calculation
• To get a molecular formula from an empirical formula:
• Divide MW by EW
• Should give a value very close to an integer.
• 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
• Used to determine empirical formula for compounds containing C/H/O
• Carbon converted to CO2
• moles of carbon = moles of CO2
• Hydrogen converted to H2O
• moles of hydrogen = 1/2 (moles of H2O)
• 2 hydrogens for every 1 H2O
• If the compound is a hydrocarbon, then you can calculate empirical formula.
• Oxygen must be determined indirectly
• Calculate the mass of C and the mass of H
• Subtract from total sample mass
• Difference is mass O
• Calculate moles of O from mass of O
• Proceed to calculate empirical formula
• Stoichiometric Calculations
• Master calculation grams of A to moles of A to moles of B to grams of B
• Allows comparison of amounts of substances based on a chemical reaction
• We will expand on this in later chapters

• Problems: 43 - 65 odd

• Limiting Reactant
• Reactant that you run out of first!
• gasoline/air
• sandwich
• To determine the limiting reactant
• calculate moles of each reactant
• divide by the stoichiometric coefficient
• choose smallest value - this is the limiting reactant
• Theoretical Yield
• How much you should get
• Actual yield - how much you do get
• less than theoretical - side reactions, physical loss, incomplete reaction, etc...
• % yield = (actual / theoretical) x 100%

• Problems: 67 - 79 odd

Learning Goals

Balance chemical equations.

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

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

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]

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

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

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

Determine the limiting reactant in a problem.

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