The Mole Concept: The Four Errors That Cost Most Marks

The central idea

A balanced equation gives ratios in moles, not in grams or litres. So every stoichiometry calculation has the same three-part shape: convert the given quantity to moles, apply the mole ratio, convert to whatever unit the answer requires.

Error one: not identifying the limiting reagent

When quantities of two reactants are given, one will usually run out first and it determines how much product forms. Calculating from the wrong one gives a larger, impossible answer.

The reliable method

1. Convert each reactant to moles.
2. Divide each by its coefficient in the balanced equation.
3. The smallest result is the limiting reagent.
4. Calculate everything from that reagent.

Dividing by the coefficient is the step that is skipped. Comparing raw mole quantities without it gives the wrong answer whenever the coefficients differ.

Error two: molarity and molality

Molality is used for colligative properties precisely because it is temperature independent, and those experiments involve changing the temperature. Being asked why is a standard question, and the temperature argument is the answer.

Note also the difference between solution and solvent: molarity uses the total volume of solution, molality the mass of solvent only. Confusing these gives answers that are close but wrong, which is harder to spot than a gross error.

Error three: empirical and molecular formula

The empirical formula gives the simplest whole-number ratio; the molecular formula gives the actual numbers. Obtaining the molecular formula requires the molar mass in addition to the composition.

1. Convert each percentage to moles by dividing by atomic mass.
2. Divide all by the smallest to get a ratio.
3. Multiply to clear fractions if needed.
4. Divide the molar mass by the empirical formula mass to get the multiplier.

Step three is where care is needed: a ratio of 1 : 1.5 must be doubled, not rounded to 1 : 2. Rounding a genuine half changes the compound entirely.

Error four: percentage yield and purity

Percentage yield compares actual product with the theoretical maximum. Purity questions require the reverse reasoning — working back from product obtained to how much of the sample was reactive.

A yield above 100 percent is impossible and signals either an arithmetic error or a wet or impure product, and saying so is the correct response to such a result rather than reporting it.

A checklist before writing the answer

• Is the equation balanced?
• Was every quantity converted to moles before any ratio was used?
• If two reactants were given, was the limiting one identified properly?
• Are the units what the question asked for?
• Is the answer physically sensible in magnitude?

Frequently asked questions

Why divide by the coefficient when finding the limiting reagent?

Because the reactants are consumed in the ratio of the coefficients. Comparing raw moles ignores that a reaction may need several moles of one reactant per mole of another.

Why is molality preferred for colligative properties?

Because it does not change with temperature, and those measurements involve temperature changes. Molarity would shift as the solution expanded or contracted.

Can the empirical and molecular formula be the same?

Yes, whenever the molecular formula is already in simplest ratio. Water is an example.

What does a yield over 100 percent mean?

An error, or a product that is impure or not fully dried. It is never a genuine result and should be reported as such.