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Lesson · Grade 11 · Chemistry

Stoichiometry: Using Mole Ratios to Predict Amounts

  • Grade 11
  • Chemistry
  • 2 min 1 s
  • British Columbia
  • BC11-SC-27

Starting from 2 H₂ + O₂ → 2 H₂O, the video shows that the coefficients count moles, not grams, then works through three steps: 4.04 g of hydrogen becomes 2.00 mol of H₂, then 2.00 mol of H₂O, then 36.0 g of water, rounded to three significant figures. It then multiplies by 6.02 × 10²³ to show that this water contains 1.20 × 10²⁴ molecules.

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In one sentence

The question this lesson answers

How much water can 4.04 grams of hydrogen make? The balanced equation is two H two plus O two makes two H two O.

Chemists count particles in moles. One mole is 6.02 times 10 to the power of 23 particles.

Transcript

What the lesson says

How much water can 4.04 grams of hydrogen make? The balanced equation is two H two plus O two makes two H two O.

Chemists count particles in moles. One mole is 6.02 times 10 to the power of 23 particles.

The coefficients count moles: 2 moles of hydrogen and 1 mole of oxygen make 2 moles of water.

They do not count grams: 2 moles of hydrogen is 4.04 grams, but 1 mole of oxygen is 32.0 grams.

So the mole ratio is 2 to 1, but the mass ratio is about 1 to 8.

Stoichiometry predicts amounts from the equation, here in three steps: grams to moles, the mole ratio, then moles to grams.

Step 1: divide by the molar mass, the mass of one mole. 4.04 grams divided by 2.02 grams per mole is 2.00 moles.

Step 2: the equation gives 2 moles of water for 2 moles of hydrogen, so 2.00 moles of water.

Step 3: multiply by the molar mass of water, 18.02 grams per mole. That gives 36.04 grams.

Significant figures are the meaningful digits. The fewest in our data is 3, so we get 36.0 grams of water.

To count molecules, multiply by 6.02 times 10 to the power of 23: 2.00 moles of water is 1.20 times 10 to the power of 24 molecules.

Remember: grams to moles, the mole ratio, then moles to grams.

Curriculum

Where this fits

British Columbia, Grade 11

Chemistry 11, The mole and stoichiometry: stoichiometric calculations using significant figuresBC11-SC-27

Previous: Grams to Moles to ParticlesNext: Gas volumes: one mole of any gas at STP

The outcomes and strands follow the British Columbia curriculum.

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