Stoichiometry & the Mole: Counting Atoms by Weighing Them

You can't count atoms one by one — a speck of matter holds trillions upon trillions. So chemists use the mole, a counting unit like a "dozen" but vastly larger, to handle atoms in practical amounts. Combined with a balanced chemical equation, the mole lets you do something powerful: predict exactly how much product a reaction will yield from a given amount of reactant. That predictive bookkeeping is stoichiometry.

This guide builds from the mole through balanced equations and mole ratios, with worked and practice questions matched to the level seen in ACT Science and chemistry at Northside Tutoring.

Why Stoichiometry Matters

Stoichiometry is the quantitative heart of chemistry and appears in ACT Science chemistry passages, often as a calculation or a proportion. The reasoning is really applied ratios. (The SAT has no science section.)

The Mole: A Chemist's Dozen

A mole is simply a count: one mole is 6.02 × 10²³ particles (Avogadro's number). Just as "a dozen" always means 12, "a mole" always means that many atoms or molecules. It's the bridge between the atomic scale and amounts we can weigh.

Molar Mass

The molar mass of a substance is its mass in grams per mole, and it's read straight off the periodic table: an element's atomic mass in grams is one mole of it. Carbon's atomic mass is about 12, so one mole of carbon weighs about 12 grams. Molar mass converts between grams and moles:

Balancing Equations

Matter is conserved, so a chemical equation must have the same number of each atom on both sides. You balance by adjusting the coefficients (the numbers in front), never the subscripts. For example:

Now there are 4 hydrogen and 2 oxygen atoms on each side — balanced.

Mole Ratios

Once balanced, the coefficients give you mole ratios — the recipe for the reaction. In 2H₂ + O₂ → 2H₂O, the ratio of hydrogen to water is 2:2 (or 1:1), and oxygen to water is 1:2. These ratios let you convert moles of one substance into moles of another.

A Stoichiometry Strategy

1. Balance the equation.
2. Convert the given amount to moles (grams ÷ molar mass).
3. Use the mole ratio from the coefficients to find moles of the target.
4. Convert back to grams if needed (moles × molar mass).

Where You'll See This — Test by Test

Stoichiometry supports ACT Science chemistry passages; the SAT has no science section and the SSAT doesn't test it. It's core high-school and AP Chemistry.

Watch the Lesson

Sometimes a diagram needs a voice. In the short video below, one of our Northside tutors walks through the core idea and works through test-style problems in real time.

Worked Example Problems

These problems are calibrated to the difficulty you'll actually see on test day. Try each one before opening the solution.

Common Mistakes to Avoid

Practice Problems — You Try

Three problems below. Work each before checking the solution.

The Northside Method — How We Teach This 1-on-1

Reading a blog is a great starting point. But there's a meaningful gap between understanding a concept and reflexively applying it under timed conditions. That gap is exactly what our tutors close.

Every Northside student works through a four-step framework:

1. Assessment. We diagnose which specific skills are slowing your student down — not just whether they "get it" in the abstract.
2. Perfect-match coach. We pair them with an elite tutor (we accept only the top 1% of applicants) whose teaching style fits how your student actually learns.
3. Bespoke plan. A roadmap built around your student's target score, target timeline, and current pacing data.
4. Data-driven adjustment. Every session ends with a check on whether the student's accuracy and speed are moving in the right direction.

And if a student meets all eligibility requirements but doesn't hit the defined score improvement? We provide 5 additional hours of cohort learning at no cost. That's the Northside guarantee — built on 25 years of measured outcomes.