CHEM 1050
Fall 2016- Prof. Torrente
NOTES ON STOICHIOMETRY
Chapter 3 in a nutshell!
1. Before doing anything else, always make sure the equation is balanced!
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When balancing, change the reactants and products COEFFICIENTS (not
subscripts!!) to achieve the same number of atoms/moles of a given element on both
sides of the equation.
If at first you don’t succeed, try again! Change the coefficients until all elements are
accounted for.
When figuring out the number of atoms/moles of a given element, make sure to take
into account both coefficients and subscripts! (coefficients multiply all subscripts in a
molecule).
2. Avogadro’s number (6.02 E23) takes us from #of atoms/molecules/particles to MOLES and
vice versa.
6.02 E23 atoms/molecules/particles = 1 mole of atoms/molecules/ particles
(2 conversion factors available, use dimensional analysis for conversion)
3. The molar mass of a substance takes us from MOLES to grams and vice versa.
 The molar mass of a substance is given by the sum of all molar masses of composing
elements (taking into account subscripts)
 A molar mass generates two conversion factors (g/mol and mol/g)
4. To calculate an empirical formula:
 If given elemental composition (percentages): most common
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If given mass composition of compound (masses): easiest
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Assume 100g of sample
Determine the grams of each element (using the percentage composition)
Calculate the moles of each element (using the element’s molar mass)
Calculate the mole ratio (divide by smallest mole number)
Use mole ratios to determine subscripts
Calculate the moles of each element (using the element’s molar mass)
Calculate the mole ratio (divide by smallest mole number)
Use mole ratios to determine subscripts
If given combustion analysis data (mass of CO2 and H2O)
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From the mass of CO2- calculate mass of C (via the moles of CO2)
From the mass of H2O- calculate mass of H(via the moles of H2O)
From the mass of C, H plus the mass of the sample: calculate mass of other elements
by subtracting mass of C, H from total mass (if any; usually oxygen)
From the masses of C, H (and O if present) calculate moles of C, H (and O if present)
Calculate the mole ratio (divide by smallest mole number)
Use mole ratios to determine subscripts
**If one mole ratio falls exactly on --.5, multiply all mole ratios by 2 to obtain whole numbers. Only
whole number ratios make physical sense!
5. To calculate a molecular formula, you must have both the empirical formula (which can be
calculated by you) and the molecular mass of the compound.
CHEM 1050
Fall 2016- Prof. Torrente
6. For stoichiometric calculations, any quantities given in grams must be transformed to MOLES.
 ALWAYS THINK MOLES!
*****MOL is the BASIS FOR ALL CALCULATIONS!****
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Coefficients give you conversion factors to use dimensional analysis to compare
two DIFFERENT materials, using the MOLE RATIO from the balanced equation!
7. To determine the limiting reactant in a reaction:
 Calculate the number of moles for both reactants (using given masses and molar
masses for each reactant)
 Use one of the reactants quantities (in moles) to determine the number of moles of the
other reactant necessary to completely use up the reactant you selected.
 If the amount of the other reactant (in moles) you need is higher than the amount of
reactant you have, that is the limiting reactant.
 Can calculate the amount of leftover reactant by using the amount of limiting reactant to
calculate how much of the other reactant gets used up, and then subtracting from the
original amount of excess reactant.
8. To calculate the theoretical yield, use reactant quantities and mole ratios to calculate the
amount of product produced (this is the maximum amount theoretically possible).
 Make sure to account for limiting reactant issues
9. To calculate the percent yield:
 Divide the actual yield (given by the problem) over the theoretical yield and multiply
times 100.