Stoichiometry and solutions for high-school chemistry: study cards over three tiers, the tricky ones, a practice quiz at three levels — Foundation, On-Level, and Challenge — and a lab where you put the mole road in order, graph a strong-acid titration curve and find the equivalence point, sort twelve substances acid / base / salt, and argue from mole counts why four grams of hydrogen outlast sixteen grams of oxygen. Work here, or print it and use a pencil — both count.
You are handed grams. The balanced equation gives you a ratio — but not a ratio of grams.
Get to moles, use the ratio, get back out. The coefficients in a balanced equation count MOLECULES, so they only ever apply to moles — applying them to grams is the single most common wrong answer in the unit.
What people get wrong
⚠️People often think…
The limiting reactant is whichever you have fewer moles of.
Not until you account for the recipe. Divide each reactant’s moles by its coefficient; the SMALLER result limits. A reaction that needs two of something can run out of it while you still have more moles than the other reactant.
Divide by the coefficient first.
⚠️People often think…
A yield over 100% means the reaction produced extra product.
Impossible — conservation of mass forbids it. Over 100% always means your product is wet, impure, or still holding solvent. It is a measurement error, and the fix is to dry the sample and weigh it again.
Over 100% means wet or impure.
⚠️People often think…
Diluting a solution reduces the number of moles of solute.
You only added water. The moles of solute are exactly what they were — they are just spread through more volume, so the molarity drops. That is why M₁V₁ = M₂V₂ works: both sides are the same unchanged moles.
Dilution changes volume and molarity. Never moles.
⚠️People often think…
A strong acid is a concentrated acid.
Two different questions. STRONG means it ionizes completely; CONCENTRATED means there is a lot of it per liter. A drop of strong acid in a bucket of water is strong and dilute; glacial acetic acid is weak and concentrated.
Strong is ionization. Concentrated is amount.
Grams in, moles, ratio, grams out
1Watch one
CH₄ + 2O₂ → CO₂ + 2H₂O. How many grams of CO₂ from 16 g of CH₄?
Molar mass of CH₄ is 16 g/mol, so 16 g is 1.0 mol. Now you are allowed to use the ratio.
The ratio CH₄ : CO₂ is 1 : 1, so 1.0 mol of CO₂ comes out.
Molar mass of CO₂ is 44 g/mol.
1.0 × 44 = 44 g of CO₂. Notice it weighs more than the methane — the oxygen came along. ✓
2Do one with me
Fill in what the numbers mean.
A mole ratio may only be applied to
Diluting a solution never changes the
Two pH units apart is a factor of
💬One sentence, then you move on
Why can a mole ratio not be applied to grams?
3Try one
You have 3 mol of A and 4 mol of B, and the equation needs 1 A to 2 B. Which limits?
I want a hint first
Divide each reactant’s moles by its own coefficient, then compare those two numbers.
💬Last one — then you're done here
Why can a strong acid be a dilute one?
Where this goes
Where this lives
Mixing a cleaner to the right strength, reading a medication concentration, why a pool test strip reports pH instead of acid concentration.
What this feeds
Next comes physics, where the bookkeeping is momentum and energy instead of moles.
Name one place you have seen a concentration printed on a label.
One card at a time — tap “Show me” to check yourself, then Next. Start at Foundation; when those feel easy, climb.
Helpful Hints
🪜 Stoichiometry and solutions — the ladder
The mole counts particles; molar mass turns grams into moles and back → the balanced equation’s coefficients are mole ratios → the mole road: grams → moles → moles → grams, ratio applied only to moles → the reactant that runs out first limits the product; percent yield is actual over theoretical → molarity is moles per liter; dilution keeps the moles → acids give H⁺, bases take it, pH is −log[H⁺], one unit per factor of ten → neutralization makes salt and water, and a titration matches moles of H⁺ to moles of OH⁻ to measure an unknown.
Count in moles; weigh in grams; molar mass is the bridge.
🤝 The whole idea
Every problem in this unit is the same problem: get to moles, use the ratio, get back out. Grams, liters and pH are just the doors you come in and leave through.
Get to moles. Use the ratio. Get back out.
⚠️ Tricky ones
NEVER apply a mole ratio to grams. Convert to moles first, every time.
The limiting reactant is not the one with fewer grams, or even fewer moles — divide each by its coefficient and the SMALLER result limits.
A yield over 100% means a wet or impure product, never extra product.
Dilution changes the VOLUME and the MOLARITY. It never changes the MOLES.
STRONG means fully ionized. CONCENTRATED means a lot per liter. A strong acid can be dilute; a weak acid can be concentrated.
pH is a LOG. Two units apart is a hundred times, not twice.
At the equivalence point MOLES of H⁺ and OH⁻ are equal — not volumes, not molarities. Count the H⁺ per molecule for diprotic acids.
🗺️ The mole road — the chart
From
To
Do this
Worked: CH₄ + 2O₂ → CO₂ + 2H₂O, 16 g CH₄
grams of A
moles of A
÷ molar mass of A
16 g ÷ 16 g/mol = 1.0 mol CH₄
moles of A
moles of B
× (coefficient of B ÷ coefficient of A)
1.0 × (1 ÷ 1) = 1.0 mol CO₂
moles of B
grams of B
× molar mass of B
1.0 × 44 g/mol = 44 g CO₂
moles
molarity
÷ liters of solution
0.5 mol ÷ 2.0 L = 0.25 M
molarity
moles
× liters of solution
0.500 M × 0.0224 L = 0.0112 mol
[H⁺]
pH
−log[H⁺]
0.001 M → pH 3
Six conversions; every problem is a chain of them.
🎯 How the test will ask
“How many grams of product form from X grams of reactant?” (the full mole road).
Two reactant masses given — “identify the limiting reactant and the mass of product.”
“A student collects 30 g when 36 g were expected. Percent yield?”
“Describe how to prepare 250 mL of 0.400 M NaCl from solid, and from a 2.00 M stock.”
“[H⁺] = 10⁻⁴ M. Find the pH and say how it compares with pH 6.”
Titration data — “find the molarity of the unknown acid.”
✅ Can you…
Convert grams to moles and back with a molar mass you computed yourself?
Run the full mole road without applying a ratio to grams?
Find the limiting reactant when the moles are equal but the ratio isn’t?
Prepare a solution from solid and by dilution?
Explain why strong and concentrated are different words?
Read a titration curve and finish the calculation?
Pick your level
Look back at anything you missed — the hint that appeared is exactly what to reread tonight.
How sure did you feel?
Lab
Work like a scientist: label the diagrams, turn the table into a graph, then write a claim with evidence and reasoning. Every lab checks itself on the spot, and hints are free.
Your practice never leaves this device. There is no account and no sign-in. Your work is saved in this browser only, and you can erase it whenever you want.
Your practice record — saved on this device
This is your record of the module on screen — it stays here and goes nowhere. Independent means you got it right on the first tap; supported means you got it after the explain-and-retry, or marked ‘I had it’ on a revealed answer. Both count, and neither is a grade. If your teacher asks, copy the row or show them this screen.
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The answer key is for a teacher: it prints only from here, for the unit on screen. Print the study packet prints the study pages and a blank quiz — never the answers.