Every calculation in this module rests on assumptions, such as a complete reaction, a pure reactant and a stated gas volume per mole. When a question asks you to explain a difference between a predicted and a measured value, you name an assumption that may not hold and say which way it moves the result.
This lesson closes concentration, gas and yield calculations by turning the numbers from earlier lessons into written reasoning.
Which assumptions are hiding in a calculation?
- The reaction goes to completion. All of the limiting reactant is used up.
- The reactants are pure. The given mass is all the named substance.
- No product is lost. Nothing is left behind or escapes in transfers.
- Only the stated reaction happens. No side reactions use up reactants.
- The stated conditions hold. A gas volume per mole applies only at the temperature and pressure given.
- The limiting reactant is identified correctly. The other reactant is truly in excess.
How do you write the explanation?
Use three parts: the assumption, what actually happened, and the effect on the number. For example: “Some of the gas escaped before it was measured, so the measured volume is lower than predicted.”
Keep it specific to the situation in the question. A reader should be able to see why the difference goes the way it does.
Worked example
The numbers are invented for practice. A student predicts that 0.120 g of magnesium will give 120 cm³ of hydrogen at room temperature and pressure with excess acid. The measured volume is 112 cm³.
Step 1, size of the difference: 120 − 112 = 8 cm³ lower. As a percentage of the prediction, 112 ÷ 120 × 100 = 93.3%.
Step 2, name an assumption: the calculation assumed that the magnesium was pure and that all the gas was collected.
Step 3, write the explanation: “The magnesium may have had an unreactive surface layer, so there was less magnesium to react than the mass suggests, which gives less gas than predicted.”
Step 4, give a second valid cause if asked: “Some hydrogen may have escaped before the gas was collected, so the measured volume is lower than predicted.”
Each answer names a cause and the direction of the change. The measured value is lower, so each explanation must also lower it.
How does the limiting reactant act as an assumption?
A prediction from one reactant assumes the other is in excess. In Mg + 2HCl → MgCl₂ + H₂, suppose 0.100 mol of magnesium meets only 0.100 mol of hydrochloric acid.
All the magnesium would need 0.200 mol of acid, so the acid runs out first and is the limiting reactant. Hydrogen then forms from the acid: 0.100 ÷ 2 = 0.0500 mol. Predicting from the magnesium would give 0.100 mol and be wrong.
What is the mistake to watch for?
A common slip is to give a vague cause that could apply to any experiment.
Mistaken answer: “The difference is because of human error.”
This names no cause and does not say whether the measured value should rise or fall.
The correction is to replace it with a named event and a direction. Ask yourself, “what physically happened, and would it make my measured number bigger or smaller?” An answer that cannot explain the direction of the difference is unlikely to earn the mark.
Check yourself
Try these, then open the answers.
1. A calculation predicts 5.60 g of product and 4.48 g is obtained. Give one assumption that may not have held and its effect.
Show answer
For example: “Some product was left behind during transfer, so the mass collected is lower than predicted.” Another valid answer is that the reaction did not go fully to completion.
2. A student weighs an impure solid and treats it as pure in a moles calculation. Is the predicted product mass too high or too low compared with the true amount, and why?
Show answer
The predicted mass is too high. The calculation counts the impurity as reactant, so it predicts more product than the real amount of reactant can make.
3. 0.100 mol of Mg reacts with 0.100 mol of HCl in Mg + 2HCl → MgCl₂ + H₂. Which reactant is limiting, and how many moles of hydrogen form?
Show answer
All the magnesium would need 0.200 mol of HCl, but only 0.100 mol is present. HCl is limiting. Moles of H₂ = 0.100 ÷ 2 = 0.0500 mol.
Where does this lead next?
Test the whole module with the mixed practice set, and log any weak steps with the mistake log and retest queue if you use it. The equation balance reasoning trainer helps confirm the starting equation.
Some students can calculate but cannot yet phrase the reasoning in a way that earns marks. That is where a teacher in online one-to-one Chemistry tuition can model and correct your written explanations.