Stoichiometry for Consecutive (Series) Reactions
Many chemical processes occur in steps where one reaction produces an intermediate that is consumed in the next.
consecutive reaction stoichiometry focuses on building a single net reaction and using its coefficients to compute mole and mass relationships.
The main quantity computed is each species amount (moles and masses) scaled from a chosen reference species.
Core definitions and formulas
Each step is first balanced, then combined using the smallest integer multipliers so that any intermediate species cancels from the overall sum.
The result is a net balanced equation with stoichiometric coefficients \(\nu_i\) used for ratio calculations.
\[
\begin{aligned}
n_{\text{ref}} &= \frac{m_{\text{ref}}}{M_{\text{ref}}} \\
n_i &= n_{\text{ref}} \cdot \frac{\lvert \nu_i \rvert}{\lvert \nu_{\text{ref}} \rvert} \\
m_i &= n_i \cdot M_i
\end{aligned}
\]
Here \(m\) is mass (g), \(n\) is amount (mol), and \(M\) is molar mass (g·mol−1).
Coefficients \(\nu_i\) come from the net reaction; absolute values keep all reported amounts nonnegative, whether a species is on the reactant or product side.
How to interpret results
Larger \(\lvert \nu_i \rvert\) means more moles of that species are associated with the same reference amount; larger \(M_i\) increases the corresponding mass for the same moles.
Typical outputs include the balanced step reactions, the net reaction after intermediate cancellation, and a table of moles and masses for every species relative to the selected reference.
Common pitfalls
- Using a reference mass with mismatched units (enter grams if molar masses are in g·mol−1).
- Forgetting that only the net reaction coefficients should be used for the final ratios.
- Providing step reactions that cannot be balanced as written (missing species or incorrect formulas).
- Interpreting results as yields or kinetics; stoichiometry alone does not include rate or conversion limits.
Micro example
Steps: \(A \rightarrow B\) and \(2B \rightarrow C\) combine to net \(2A \rightarrow C\).
If \(m_A = 10\,\text{g}\) and \(M_A = 50\,\text{g·mol}^{-1}\), then \(n_A = 0.20\,\text{mol}\) and \(n_C = 0.10\,\text{mol}\).
With \(M_C = 80\,\text{g·mol}^{-1}\), the predicted mass is \(m_C = 8.0\,\text{g}\).
Use this approach to convert between amounts in multi-step pathways where intermediates should cancel in the net equation.
Avoid using it for problems that require limiting reactant checks, percent yield, side reactions, or time-dependent behavior; a next-step concept is limiting reactants and yield analysis, or (for mechanisms) rate laws.
