Chemistry 1364

General Chemistry I

Chapter 3 - Stoichiometry

- Chemical Equations
- 2H
_{2}+ O_{2}--> 2H_{2}O - H
_{2}and O_{2}are reactants. - H
_{2}O 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 O
_{2}(similar to algebra.) - Balancing - same # of each type of atom on each side of reaction.
- CANNOT change subscripts
- Change only SC
- Periodic table to help predict reaction.
- 2K + 2H
_{2}O --> 2KOH + H_{2} - use to predict reaction of Na, Rb, etc.
- C/H/O - combustion
- form CO
_{2}+ H_{2}O - Combination
- form 1 product
- Decomposition
- from 1 reactant
- Atomic weight (AW)
- weighted average of all isotopes observed in nature.
- a.m.u. scale - based on mass of
^{12}C = 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)
- Avogadro's number
- # of atoms of
^{12}C 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.
- 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 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 CO
_{2} - moles of carbon = moles of CO
_{2} - Hydrogen converted to H
_{2}O - moles of hydrogen = 1/2 (moles of H
_{2}O) - 2 hydrogens for every 1 H
_{2}O - 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
- 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: 1-7 odd

Problems: 9-13 odd

**NOTE: You MUST master skill of calculating a molecular weight from the atomic weights!**

Problems: 15 41 odd

Problems: 43 - 65 odd

Problems: 67 - 79 odd

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.