How Can We Determine the Exact Proportions in Which Two Chemical Substances React?
1. The challenge
In many professions it is necessary to calculate quantities with precision. A tailor estimates the amount of fabric needed to make a garment, and an accountant calculates deductions from a worker’s annual income.
Something similar happens in chemistry, but with an even greater degree of accuracy. Chemists must know how much of each substance is required for a reaction to occur properly and to predict how much product will be formed.
The set of methods used to calculate these relationships is called stoichiometry. In this experiment, we will observe how two chemical solutions react to form a solid precipitate. From the proportions used and the amount of product formed, we will be able to identify the correct relationship between the reactants.
2. Importance in the real world
Stoichiometry is an essential tool in chemistry and in many areas of industry because it allows scientists and engineers to:
- Calculate how much reactant is needed to produce a substance
- Estimate how much product will be obtained
- Avoid wasting materials
- Optimize industrial processes
These calculations are fundamental in activities such as:
- The production of medicines
- The manufacture of fertilizers
- The development of industrial materials
- Water treatment processes
In all these cases, knowing the exact reaction proportions is essential for working efficiently and safely.
3. Mental model of the experiment
In this experiment, two solutions are mixed:
- Barium chloride (BaCl₂)
- Sodium sulfate (Na₂SO₄)
When these substances react, they form an insoluble solid called barium sulfate (BaSO₄), which appears as a white precipitate. The chemical reaction is:
The amount of precipitate formed depends on the amount of reactants available. If one of the reactants is used up first, the reaction stops. This substance is called the limiting reactant.
By varying the proportions of each solution in different test tubes, we can observe how the amount of precipitate changes and discover the optimal proportion between the reactants.
4. Common misconception
“If we mix more reactants, we will always obtain more product.”
This is not always true. In many chemical reactions, one reactant is completely consumed before the other.
When this happens, the reaction stops even though some of the other reactant is still present in excess.
The substance that is consumed first is called the limiting reactant, and it determines the maximum amount of product that can be formed.
5. Expanding the challenge
In this experiment, six test tubes are prepared with different proportions of BaCl₂ and Na₂SO₄ solutions.
| Tube | BaCl₂ 0.5 M (mL) | Na₂SO₄ 0.5 M (mL) |
|---|---|---|
| 1 | 0 | 5 |
| 2 | 1 | 4 |
| 3 | 2 | 3 |
| 4 | 3 | 2 |
| 5 | 4 | 1 |
| 6 | 5 | 0 |
After mixing the solutions and waiting a few minutes, the solid barium sulfate settles at the bottom of each tube.
By measuring the height of the precipitate, you can estimate the relative amount of product formed.
If you represent these values on a graph, you will observe that:
- The amount of precipitate increases gradually
- It reaches a maximum point
- Then it decreases again
The highest point on the graph indicates the ideal stoichiometric proportion, meaning the exact ratio in which both reactants are completely consumed.
6. Scientific microhistory
At the end of the 18th century, the French chemist Antoine Lavoisier demonstrated that matter is neither created nor destroyed during a chemical reaction. This principle, known as the law of conservation of mass, made it possible to understand that reactions occur according to definite proportions.
Decades later, scientists such as Joseph Proust and John Dalton discovered that elements combine in constant mass ratios.
These ideas gave rise to stoichiometry, the tool that allows chemists to calculate precisely the quantities of reactants and products in any chemical reaction.
7. Final question
If we can calculate the proportions of a chemical reaction with precision…
Would it be possible to predict exactly how much matter is needed to produce tons of an industrial material or even a medicine?
