Investigation 7: Stoichiometry
Continuous variation experiments mix two reactants in different proportions while keeping total volume constant, and the mixture that produces the most product reveals the reaction's mole ratio.
You make a series of mixtures of reactant A and reactant B, always the same total volume, same starting concentrations, but sliding the ratio from mostly A to mostly B. Each mixture produces some observable signal tied to product amount: a temperature rise, a precipitate mass, a color intensity. Plot that signal against the mole fraction (or volume) of A. The graph rises, peaks, then falls. Past the peak, you have excess of one reactant and not enough of the other, so product stops climbing no matter how much more of the excess reactant you dump in. The limiting reactant caps how much product can form.
The peak is the point where A and B are present in exactly the ratio the balanced equation demands, with nothing left over. Since both stock solutions start at the same concentration, moles are proportional to volume, so you can read the stoichiometric ratio straight off the volumes at the peak. No concentration calculations needed, just a volume ratio.
This lab is really a hands-on proof that stoichiometry controls yield, tying directly into limiting reactant reasoning from earlier units and into the error analysis and graphing practices this unit emphasizes: finding a smooth peak from scattered data points, and recognizing that the true maximum may fall between two tested mixtures.
Key terms
- Continuous variation method
- A technique (also called Job's method) that varies the ratio of two reactants while holding total volume constant to find stoichiometry from peak yield.
- Mole fraction
- The ratio of moles of one component to the total moles of all components in a mixture.
- Limiting reactant
- The reactant that runs out first, stopping further product formation.
- Stoichiometric ratio
- The mole ratio of reactants (or products) given by the coefficients of the balanced chemical equation.
Watch it explained
- How it worksGraph product yield against the mole fraction of one reactant and find the peak.
- How it worksWalk me through using a continuous variation experiment to find the stoichiometric ratio.
- Worked exampleFind the mole ratio of a reaction whose yield peaks when 6 mL of A is mixed with 4 mL of B.
- Common mistakeWhy isn't the maximum product always at a one to one mixture of the two solutions?
Check yourself
End of topic.