Stoichiometry
The word Stoichiometry comes from the Greek word “stoichio” meaning element and “meter” meaning measure. Stoichiometry
is the branch of chemistry concerning quantities of substances that react in or are produced in chemical reactions.
Review Reactions
Classify the Reaction, Predict the Products, and Balance the Chemical Equation
(1) sodium reacts with sulphur
(2) zinc chloride reacts with aluminum
(3) sodium hydroxide reacts with oxalic acid
Mole Ratios
The moles of substances in a chemical reaction are always in proportion as determined from the balanced chemical equation.
ex. Given the balanced reaction:
O2 + 4HBr
2H2O + 2Br2
From the coefficients of the equation, it can be seen that the chemicals must be combined in the following proportions: for every
one mole of O2 reacting there must be four moles of HBr reacting and there will be two moles of H2O produced and two moles of
Br2 produced.
1 mol O2 = 4 mol HBr = 2 mol H2O = 2 mol Br2
Given the moles of one substance in a reaction, the moles of any other substance can be determined by multiplying by a mole
ratio. A mole ratio is a conversion fraction that shows the ratio of coefficients of two substances from the balanced chemical
equation. The conversion fraction is written such that the coefficient of the given substance is in the denominator and the
coefficient of the substance to be determined is in the numerator (in order for the units to cancel appropriately).
ex. if 2.00 moles of HBr react, determine the moles of O2 reacting.
2.00 mol HBr x
1 mol O2
4 mol HBr
0.500 mol O2
MOLE RATIO
ex. if 0.080 moles of H2O are produced, determine the moles of HBr reacting
ex. if 15 mol of Br2 are produced, determine the moles of O2 reacting
Given the balanced reaction:
Ni2(SO4)3 + 2Na3PO4
2NiPO4 + 3Na2SO4
ex. if 0.050 mol of NiPO4 are produced, determine the moles of Ni2(SO4)3 reacting
ex. if 2.40 mol of NiPO4 are produced, determine the moles of Na2SO4 produced
ex. if 10.2 mol of Na3PO4 are reacted, determine the moles of Na2SO4 produced
Stoichiometry Calculations involving Mass
ex. Sodium fluoride is reacted with calcium nitrate. If 8.40 g of sodium fluoride are present, what mass of calcium nitrate would
be required in the reaction? What would be the mass of each of the products?
(1) Determine a balanced chemical equation: Write the chemical formulas for each substance, being sure to balance the charges
if needed. Add coefficients so that the number of atoms of each element is the same of both side of the reaction.
(2) Draw a table with three rows labeled moles (mol), mass (g), and molar mass (g/mol) with a column below each chemical.
(see below)
(3) Determine the given information and enter it below the proper chemical in the mass row. Determine what masses the
question is asking for and enter question marks below the proper chemical in the mass row.
(4) Calculate the molar mass of each chemical (remember, coefficients are not used in calculating molar mass).
(5) Use the given mass to calculate the number of moles present.
(6) Use the moles that were calculated to determine the moles of other chemicals in the reaction, using the correct mole ratio.
(7) Use the moles to determine the mass each chemical.
Reaction:
moles (mol)
mass (g)
molar mass (g/mol)
_____________ + ___________
____________ + _____________
Percent Yield
In laboratory stoichiometry experiments, the amount of product predicted by stoichiometry is often not obtained. Side reactions,
contamination, incomplete reaction, or loss of product can cause the amount of product actually obtained (the “actual mass”) to
be lower than the amount of product predicted by stoichiometry (the “theoretical mass”). Percent Yield is calculated by dividing
the actual mass a given product by the theoretical mass of that product.
% yield
actual mass
x 100%
theoretical mass
Percent yield will always be equal to or less than 100%. Percent yield can vary for a reaction depending on the reaction
conditions (i.e. different experimental conditions can increase or decrease the percent yield). The actual yield for other products
in a reaction can be calculated using the percent yield and the theoretical mass.
ex. Aluminum sulphide can be decomposed to form aluminum and sulphur. If 24.0 g of aluminum sulphide reacts, calculate the
theoretical yield of each of the products. If the reaction actually produces 7.40 g of aluminum, determine the percent yield for
the reaction. What mass of sulphur would actually be obtained?
Reaction:
moles (mol)
mass (g)
molar mass
(g/mol)
____________
____________ + ____________
Limiting and Excess Reactions
It is common when preparing a chemical reaction to add more of one of the reactants than is actually necessary (more than would be
required by stoichiometry). Adding extra of one reactant can help to ensure that the other reactant is entirely used up. In this situation.
the reactant with a greater amount of moles would be in excess, so it is called the excess reactant and the reactant with a smaller
amount of moles would be limiting, so it is called the limiting reactant.
Limiting and Excess Reactions with Moles
For each reaction, determine which reactant is limiting and which is in excess. The limiting reactant can be found by taking the moles
of each reactant and dividing by the coefficient for that reactant. The substance with the lower quantity is limiting and the substance
with the greater quantity is in excess. The limiting reactant is used to calculate the moles and mass of product formed.
ex. Mg + S
MgS
(a)
Mg
moles (mol)
+
1.0
S
MgS
1.5
mass (g)
molar mass (g/mol)
(a)
Mg
moles (mol)
+
0.80
S
MgS
0.40
mass (g)
molar mass (g/mol)
ex. 4Na + O2
2Na2O
(a)
4Na
moles (mol)
+
10.0
O2
2Na2O
2.00
mass (g)
molar mass (g/mol)
(b)
4Na
moles (mol)
+
12.0
O2
2Na2O
5.00
mass (g)
molar mass (g/mol)
(c)
4Na
moles (mol)
+
1.00
O2
2Na2O
0.250
mass (g)
molar mass (g/mol)
ex. 2Al + 3F2
2AlF3
(a)
2Al
moles (mol)
+
2.0
3F2
2AlF3
4.0
mass (g)
molar mass (g/mol)
(a)
2Al
moles (mol)
mass (g)
molar mass (g/mol)
0.24
+
3F2
0.33
2AlF3
Limiting and Excess Reactions with Mass
If the mass of both reactants are given, begin by calculating the moles of each reactant. Determine which reactant is limiting and
which is in excess (by dividing the moles of each reactant by the coefficient and finding which is lower). Use the moles of the
limiting reactant to calculate the moles and mass of product formed. To determine the mass of the excess reactant used in the
reaction, use the moles of the limiting reactant to calculate the moles of the excess reactant used and then the mass of the excess
reactant used. To calculate the mass of the excess reactant remaining, subtract the mass of the excess reactant used from the mass
of the excess reactant available/given.
ex. 18.8 g of beryllium fluoride are reacted with 12.4 g of magnesium.
(a) Which reactant is limiting and which is excess?
(b) What is the mass of each of the products?
(c) What mass of the excess reactant is used in the reaction and what mass of the excess reactant remains after the reaction?
Reaction:
_____________ + ___________
____________ + _____________
moles (mol)
mass (g)
molar mass (g/mol)
ex. Zinc reacts with phosphorus to produce zinc phosphide.
51.0 g of zinc are reacted with 19.2 g of phosphorus.
(a) Which reactant is limiting and which is excess?
(b) What is the theoretical mass of the product?
(c) What mass of the excess reactant is used in the reaction and what mass of the excess reactant remains after the reaction?
(d) Calculate the actual mass of product obtained if the reaction has an 80.0% yield.
Reaction:
moles (mol)
mass (g)
molar mass
(g/mol)
____________ + ____________
___________