Stoichiometry problems are commonly about two substances involved in a chemical reaction.
It is common for reactants to be added to the reaction mixture as solutions of known concentration.
Below is a summary of the approach that BestChoice uses to solve stoichiometry problems.
Identify the Known substance.
Identify the Unknown substance.
Assign symbols to the given data and to the unknown quantity.
Connect the Known and Unknown by identifying any intermediate quantities to be calculated.
Consider the example where substance A and B react according to
aA + bB
cC + dD
a, b, c and d are numbers required to balance the equation (stoichiometric coefficients).
Identify the mathematical relationships required to do each step of the calculation.
Carry out the calculation
It is common for reactants to be added to the reaction mixture as solutions of known concentration.
Below is a summary of the approach that BestChoice uses to solve stoichiometry problems.
Identify the Known substance.
Enough information is given to calculate the amount in moles (n) of the Known.
The amount in moles can be calculated EITHER from the mass (m) of a pure substance
OR from the concentration (c) and volume (V) of a solution.
n = cV
The amount in moles can be calculated EITHER from the mass (m) of a pure substance
OR from the concentration (c) and volume (V) of a solution.
n = cV
Identify the Unknown substance.
Assign symbols to the given data and to the unknown quantity.
Be sure to include the substance to which each symbol refers, for example c(NaOH).
This helps you to connect the data to any mathematical relationships that you have learned.
This helps you to connect the data to any mathematical relationships that you have learned.
Connect the Known and Unknown by identifying any intermediate quantities to be calculated.
Relating amounts in moles of two substances can be done either directly as shown at the right or through amount in moles of reaction as below.
Consider the example where substance A and B react according to
aA + bB
cC + dDa, b, c and d are numbers required to balance the equation (stoichiometric coefficients).
Known | Use | Calculate |
| Substance A c(A) and V(A) | n = cV | n(A) moles of A |
Use | Calculate | Use | calculate |
| divide by a | n(reaction) | multiply by b | n(B) moles of B |
Use | Unknown |
V(B) | Substance B c(B) |
Note that some given data [like V(B)] may not be used until late in the calculation.
Identify the mathematical relationships required to do each step of the calculation.
The three relationships (or rearranged forms of these) shown on this page are the most common for stoichiometry calculations.
Carry out the calculation