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Teach & case

Help a near-peer reason through the idea — when you explain it, you understand it best — then read the real decision it comes from.

A case · you decide

Halden and the doubled recipe

Halden — a student scaling up a lab reaction and unsure how to read the balanced equation

The idea in play: stoichiometry — a balanced equation gives MOLE ratios, not gram/mass ratios.

Halden needs to react hydrogen with nitrogen to make ammonia: N₂ + 3H₂ → 2NH₃. He reads "1 to 3" and grabs 1 gram of N₂ and 3 grams of H₂. Something feels off when the numbers don't work out.

The coefficients (1, 3, 2) are a MOLE ratio: 1 mole N₂ reacts with 3 moles H₂. Moles count particles, not grams — and a mole of N₂ weighs 28 g while a mole of H₂ weighs about 2 g.

So "1 to 3 by moles" is NOT "1 g to 3 g." By mass it's 28 g of N₂ to 6 g of H₂. Reading the ratio as grams silently changes the whole reaction.

The equation says N₂ + 3H₂. Does that mean 1 gram of N₂ per 3 grams of H₂?

Halden has 2 moles of N₂. How much H₂ does he need — and why not just "3"?

Work a mole-ratio problem yourself →

When two ideas meet: 2 cases with two people

A case · you decide

Halden and Nerys: from a label to a mass

Halden & Nerys — a student who counts in moles, and a friend who reads concentrations

The idea in play: concentration times volume gives moles, and moles times molar mass gives grams.

Nerys needs 250 mL of 0.40 M salt solution. Halden has to weigh out the sodium chloride, 58.44 g per mole.

0.40 M means 0.40 moles in every litre. 250 mL is a quarter of a litre.

How much sodium chloride should Halden weigh out?

Open the Calculation Lab →

A case · you decide

Basil and Perrin: squeeze the equilibrium

Basil & Perrin — a student who predicts equilibrium shifts, and a friend who thinks about gas pressure

The idea in play: raising the pressure of a gas equilibrium shifts it toward the side with fewer moles of gas.

N₂ + 3H₂ ⇌ 2NH₃ has come to equilibrium in a sealed cylinder. Perrin pushes the piston in, halving the volume.

Perrin counts the gas molecules on each side: 1 + 3 = 4 on the left, 2 on the right.

Which way does the equilibrium shift?

Open the Calculation Lab →