Stoichiometry Triangle Chart & T-Chart
Start. Switch. End. That’s the triangle. The t-chart lives inside Switch. Together, they’re a system for setting up every stoichiometry problem — mole-to-mole, mass-to-mass, and percent yield — so unit cancellation is visual, not guesswork. Here’s how each piece works and how to put them together.
The triangle diagram looks simple. Three sections, two triangles, some lines. What trips students up isn’t drawing it — it’s knowing what goes where, especially inside the Switch section. That’s where the t-chart lives, and the t-chart is where most unit errors happen. Get both tools straight and stoichiometry stops feeling like guessing.
What This Guide Covers
How the Triangle Chart Works
The triangle isn’t decorative. It’s a visual organizer that forces you to separate three distinct things: what you’re starting with, the conversion you’re applying, and what you’re solving for. That separation is the whole point.
Each problem uses two triangles. The left triangle is the Start side. The right triangle is the End side. Between them — in the center — is Switch. That’s where your conversion factor lives. The horizontal lines inside each triangle are where you write numbers and units, row by row, from the bottom up as the problem builds.
Start (left triangle): Write your given value at row 1 (bottom). The unit is what the problem hands you — grams, moles, etc. Work up the triangle as you set up the calculation.
Switch (center, between triangles): The conversion factor goes here as a t-chart fraction. Top = the unit you want. Bottom = the unit you’re canceling. This is the mole ratio from the balanced equation, or the molar mass conversion, depending on the problem type.
End (right triangle): The final answer goes here. Row 3 (top) is the calculated result with correct units. Your work accumulates up the triangle — by the top, you should be able to read the full setup without referring back to the problem.
The reason teachers use the triangle format is unit cancellation. When you write units explicitly in every row — and the t-chart fraction shows what cancels and what stays — you can catch errors by inspection. If the unit on the bottom of the Switch t-chart doesn’t match the unit on row 1 of Start, the setup is wrong before you ever touch a calculator.
The T-Chart Format Explained
The t-chart is just a fraction written with a horizontal line separating numerator from denominator. It looks like the letter T. That fraction is your conversion factor — and how you write it determines whether units cancel correctly.
One rule: whatever unit you want to end up with goes on top. Whatever unit you want to eliminate goes on the bottom. That’s it. If you’re converting moles of substance A to moles of substance B, the mole ratio from the balanced equation tells you the numbers. The direction of the fraction is your choice — and you choose based on what you need to cancel.
Notice that the molar mass t-chart flips depending on direction. Going from grams to moles: molar mass goes on the bottom (you’re canceling grams). Going from moles to grams: molar mass goes on top (you’re producing grams). The number doesn’t change. The position does.
Mole Ratios Come From the Balanced Equation — Not From Memory
The coefficients in the balanced equation are the mole ratio. If the equation shows 2 KClO₃ → 2 KCl + 3 O₂, then the mole ratio of O₂ to KCl is 3:2. To convert moles of KCl to moles of O₂, the t-chart top gets 3 mol O₂ and the bottom gets 2 mol KCl. Those numbers come directly from the equation — you don’t estimate or look them up.
Molar mass numbers come from the periodic table. Add up the atomic masses of every atom in the compound. That sum, in grams per mole, is what goes in the molar mass t-chart. For KClO₃: K (39.1) + Cl (35.5) + 3×O (48.0) = 122.6 g/mol.Mole-to-Mole Problems
This is the simplest type. You’re given moles of one substance and asked for moles of another. One t-chart in the Switch section. No molar mass conversion needed because you’re already working in moles on both sides.
Look at the unit in row 1 of Start. Look at the unit on the bottom of the Switch t-chart. They must be identical — same substance, same unit. If they don’t match, the t-chart is set up backwards. Flip it before doing any arithmetic.
Mass-to-Mass Problems
Three t-charts. That’s what distinguishes mass-to-mass from mole-to-mole. You can’t apply a mole ratio directly to grams — the mole ratio only works with moles. So you have to convert grams to moles first, apply the ratio, then convert back to grams. The triangle tracks each of those three steps.
Problem: How many grams of KCl are produced from 3.26g of K? (2K + Cl₂ → 2KCl)
T-chart 1: convert grams K to moles K. Top = 1 mol K. Bottom = 39.1 g K (molar mass of K). This cancels the grams K unit.
T-chart 2: apply the mole ratio. Coefficient of KCl = 2, coefficient of K = 2. Top = 2 mol KCl. Bottom = 2 mol K. Mol K cancels.T-chart 3: convert moles KCl to grams KCl. Molar mass of KCl = 39.1 + 35.5 = 74.6 g/mol. Top = 74.6 g KCl. Bottom = 1 mol KCl. Mol KCl cancels.
Multiply across: 3.26 × (1/39.1) × (2/2) × (74.6/1) = 6.23 g KCl. Units that remain: grams KCl — which is exactly what the problem asked for.
What Each Row in the Triangle Holds
- Row 1 (bottom): the given value and unit — what the problem stated
- Row 2 (middle): running intermediate after t-chart 1 — value with new unit
- Row 3 (top): the answer — final value with the target unit
The triangle rows correspond to where you are in the chain of t-charts. Each t-chart moves you one row up the triangle.
Why the 2/2 Ratio Still Matters
When the mole ratio is 1:1 or equal coefficients (like 2:2), students sometimes skip writing it. Don’t. First, a 2:2 ratio simplifies to 1:1 — which you should write explicitly so your teacher can check the setup. Second, problems where the ratio isn’t 1:1 follow the same format. Building the habit with every problem means you won’t skip a real ratio when it matters.
Percent Yield Problems
Percent yield is a two-part problem. Part A is a standard stoichiometry calculation — usually mass-to-mass — to find the theoretical yield. Part B uses that result and the actual lab yield to calculate a percentage. The triangle handles Part A. The percent yield formula handles Part B.
Theoretical yield = what the stoichiometry calculation says you should get if the reaction goes perfectly. This is what Part A gives you.
Actual yield = what you measured in the lab (or what the problem tells you was collected). This is always less than or equal to theoretical yield. If the problem gives you a lab result, that’s your actual yield.
Percent yield = (actual / theoretical) × 100. A percent yield over 100% means an error in measurement or setup — that doesn’t happen in a correct calculation.
Problem: 45.0g KClO₃ → KCl + O₂. Lab gives 15g of O₂. What is percent yield?
Part A uses the triangle. Set up three t-charts: grams KClO₃ → moles KClO₃ (molar mass = 122.6 g/mol), moles KClO₃ → moles O₂ (ratio from balanced equation: 2 KClO₃ : 3 O₂), moles O₂ → grams O₂ (molar mass O₂ = 32.0 g/mol).
Part A calculation: 45.0 × (1/122.6) × (3/2) × (32.0/1) = 17.6g O₂ theoretical yield.Part B — percent yield: (15g actual / 17.6g theoretical) × 100 = 85.2%.
The triangle diagram is only used for Part A. Part B is a single formula applied directly to the numbers. Don’t try to draw a triangle for the percent calculation — it doesn’t belong there.
Matching Problem Type to Setup
Different stoichiometry problems require different numbers of t-charts. The table below shows what each problem type needs so you can identify what the Switch section must contain before you start writing anything.
| Problem Type | Given → Desired | T-Charts Needed | Triangle Rows Used |
|---|---|---|---|
| Mole-to-mole | mol A → mol B | 1 (mole ratio only) | Row 1 → Row 3 (two rows) |
| Mole-to-mass | mol A → g B | 2 (mole ratio + molar mass of B) | All three rows |
| Mass-to-mole | g A → mol B | 2 (molar mass of A + mole ratio) | All three rows |
| Mass-to-mass | g A → g B | 3 (molar mass A + ratio + molar mass B) | All three rows across both triangles |
| Percent yield | g A → theoretical g B, then % yield | 3 for Part A + formula for Part B | Full triangle for Part A only |
The mole ratio comes from the coefficients in the balanced equation. If the equation isn’t balanced, the coefficients are wrong, which means the ratio is wrong, which means the answer is wrong — even if every other step is correct. Before writing a single t-chart, confirm the equation is balanced. Count atoms on both sides. If they don’t match, fix it.
Common Mistakes That Cost Points
Putting the Mole Ratio Upside Down
Writing the coefficient of the given substance on top cancels the wrong unit. The unit you want to get rid of goes on the bottom — not the top. Check: does the bottom unit match what you started with?
Write the Target Unit First — Then Build the Fraction
Ask: what unit do I need at the end of this t-chart? Write that on top. Then ask: what unit am I currently holding? Write that on the bottom. Numbers follow the units — don’t lead with the numbers.
Using the Wrong Molar Mass
Using the molar mass of one substance for a different substance in the same problem is extremely common in multi-step mass-to-mass problems. Each conversion needs the molar mass of the specific substance being converted at that step.
Label the Substance in Every T-Chart
Write the substance name or formula next to every unit in every t-chart. “mol” alone doesn’t tell you which substance. “mol KClO₃” does. This prevents swapping molar masses between t-charts 1 and 3.
Skipping Part A on Percent Yield Problems
Dividing the actual yield by a number you guessed or estimated instead of a number calculated from stoichiometry. The theoretical yield must come from the triangle calculation. There’s no shortcut.
Treat Part A and Part B as Separate Problems
Finish the full stoichiometry calculation first. Write down the theoretical yield with units. Then — and only then — apply the percent yield formula. Keep the two parts visually separate on the paper so neither gets tangled up in the other.
Using the Wrong Coefficients From the Equation
Pulling the coefficient of a different pair of substances, or using subscripts (the small numbers in formulas) as if they were coefficients. Subscripts are not stoichiometric coefficients.
Identify the Two Substances in the Problem, Then Read Their Coefficients
Highlight or underline the given substance and the desired substance in the balanced equation. Read their coefficients — only those two. Write those numbers in the mole ratio t-chart and ignore the rest of the equation for that step.
Frequently Asked Questions
Need Help With Chemistry Assignments?
Stoichiometry problem sets, percent yield worksheets, balancing equations, and full chemistry assignment support — our science team works with students at every level.
Chemistry Homework Help Get StartedBefore You Start the Next Problem
The triangle and t-chart work together — but only if you fill in the units before you fill in the numbers. Every t-chart needs units written explicitly: not just “39.1” but “39.1 g K / 1 mol K.” Not just “3/2” but “3 mol O₂ / 2 mol KCl.” When units are written out in full, wrong setups become obvious by inspection before the calculator ever comes out.
Check your balanced equation before step one. Label every row. Write units in every cell of every t-chart. Confirm unit cancellation visually before multiplying. And on percent yield problems, treat Part A and Part B as two completely separate calculations — one uses the triangle, one uses the formula. Don’t blur them.
The triangle diagram is a habit-building tool. The students who use it well aren’t necessarily faster at arithmetic. They’re just more systematic about where each piece of information goes. That’s what keeps the setup clean and the answer right.