Chemical Reactions · Grade 10

How do you predict how many grams of product a given mass of reactant will give?

In three steps. Convert the reactant's mass to moles, use the coefficient ratio in the balanced equation to cross to moles of product, then convert back to grams. The equation is a recipe written in moles, and the mole is what lets you enter and leave it with a balance.

Learning objectives

A recipe written in moles

In 2H2 + O2 → 2H2O the coefficients say that two moles of hydrogen react with one mole of oxygen to give two moles of water. That is a ratio between particle counts, not between grams.

So the route from mass to mass always passes through moles: grams of the reactant, divide by its molar mass, multiply by the ratio from the equation, multiply by the product's molar mass.

Anyone who tries to shorten this by multiplying grams by the coefficient ratio gets a wrong answer almost every time, because coefficients count particles and different particles have different masses.

The limiting reactant

When you mix amounts that are not exactly in the equation's ratio, one reactant runs out first. That one decides how much product forms, and the other sits in the flask in excess without reacting.

The plain picture: ten buns and six burgers means six meals. The buns are in excess and make no difference to the answer.

To find it, work out how much product each reactant alone would give and take the smaller of the two. The smaller answer is what the materials can actually supply.

Why the laboratory gives less

The theoretical yield is what the calculation predicts. The actual yield is what ends up on the balance, and it is almost always lower.

The reasons are practical: product left clinging to the glassware, a reaction that did not run to completion, side reactions making something else, and losses during filtering and transfer.

Percentage yield is actual divided by theoretical, times a hundred. Above a hundred per cent is impossible — that result means the measurement includes something extra, usually water that was not dried off.

Worked examples

  1. How many grams of water form from 4 g of hydrogen with oxygen in excess? (2H2 + O2 → 2H2O)

    1. The molar mass of H2 is 2, so 4 g is 2 moles
    2. The equation's ratio is 2 moles hydrogen to 2 moles water, so one to one
    3. That gives 2 moles of water
    4. Molar mass 18, so 2 times 18

    Answer: 36 g of water

  2. You mix 2 moles of hydrogen with 2 moles of oxygen. Which is limiting?

    1. The ratio required is 2 hydrogen to every 1 oxygen
    2. 2 moles of hydrogen need only 1 mole of oxygen
    3. There are 2 moles of oxygen, so 1 mole is in excess

    Answer: Hydrogen is limiting; one mole of oxygen is left over

  3. The calculation predicted 36 g and the laboratory weighed 30 g. What is the percentage yield?

    1. Yield = actual divided by theoretical, times 100
    2. 30 divided by 36
    3. About 0.83

    Answer: About 83 per cent

Common mistakes

Multiplying grams by the coefficient ratio
Coefficients count particles, not grams. You have to go to moles, apply the ratio, and only then come back to grams.
Calculating from whichever reactant there is more of
The product is set by whichever runs out first. Work it out from both and take the smaller answer.
Reporting a yield above one hundred per cent
You cannot get more than the atoms allow. Such a result means the weighed product contains something else as well, usually water.

What to remember

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