Creating pathways to solve problems
A key starting point to solving stoichiometry problems is to identify the substance for which enough information is given to calculate the amount in moles. Knowledge of the amount in moles of one substance produced or consumed in a chemical reaction (the Known) allows calculation of the amounts in moles of other substances produced or consumed (the Unknowns) in the same chemical reaction.
The scheme shows how amount in moles of Known can be calculated from a given mass, and then related to either the concentration or the volume of a solution of the Unknown, provided one of these is given..
| Known substance A | Unknown substance B | |||||
| m(A) mass of A |  Step 1 | n(A) moles of A | Step 2 | n(B) moles of B | V(B) Step 3 | c(B) concentration of B |
| c(B) | V(B) volume of B |
In the problems in this module the Known is used in solution. The scheme shows that only Step 1 differs from the steps shown above. In these cases, the amount in moles of Known is calculated using the given concentration of the solution and the volume required for complete reaction.
| Known substance A | Unknown substance B | |||||
| c(A) and V(A) concentration and volume of A | Step 1 | n(A) moles of A | Step 2 | n(B) moles of B | V(B) Step 3 | c(B) concentration of B |
| c(B) | V(B) volume of B |