Mass / Mole Stoichiometry
Use the coefficients from your balanced equation
Uses the mole ratio from your balanced equation. Methodology & sources →
Stuck on a reaction problem? The General Chemistry Workbook has a full stoichiometry chapter with worked examples and an answer key.As an Amazon Associate we earn from qualifying purchases.
View on Amazon →The Mole-Ratio Bridge
Balance the equation first — its coefficients are the mole ratios. Then convert the known amount to moles, cross to the wanted substance using the ratio of coefficients, and convert back:
For example, in 2 KClO₃ → 2 KCl + 3 O₂, the O₂ : KClO₃ ratio is 3 : 2. Need to balance the equation, or find two reactants' limiting amounts? See How to Approach Stoichiometry and the Limiting Reagent Calculator.
Worked Example — Mass-to-Mass Stoichiometry
Question: Iron reacts with chlorine gas: 2 Fe(s) + 3 Cl₂(g) → 2 FeCl₃(s). How many grams of FeCl₃ form when 38.5 g of Cl₂ reacts with excess iron?
Step 1 — moles of Cl₂ (M = 70.90 g/mol): n = 38.5 ÷ 70.90 = 0.543 mol
Step 2 — mole ratio from the balanced equation (2 FeCl₃ per 3 Cl₂): n(FeCl₃) = 0.543 × (2/3) = 0.362 mol
Step 3 — convert to mass (M(FeCl₃) = 162.20 g/mol): m = 0.362 × 162.20 = 58.7 g
Answer: 58.7 g of FeCl₃.
Common Mistakes
- Using an unbalanced equation. The mole ratio comes directly from the balanced equation's coefficients — an unbalanced equation gives a wrong ratio even if every other step is correct.
- Ratio direction. The mole ratio multiplier is (moles of what you want) ÷ (moles of what you're given), taken from the equation — here 2 (FeCl₃) over 3 (Cl₂), not the other way round.
- Skipping the mole step. You cannot go directly from grams of one substance to grams of another — mass ratios are not the same as mole ratios, because molar masses differ.
Frequently Asked Questions
Convert the known mass to moles (n = m/M), multiply by the mole ratio (coeff. wanted ÷ coeff. known), then convert back to grams (m = n×M).
The ratio of the coefficients of two substances in the balanced equation. It is the bridge that lets you cross from moles of one substance to moles of another.
Yes — this calculator works from a balanced equation's coefficients. If you're unsure how to balance an equation, see the Balancing Chemical Equations guide first; an unbalanced equation will give an incorrect mole ratio even if the rest of the calculation is set up correctly.
The same three-step method (mass → moles → mole ratio → moles → mass) works between any two substances in the equation — just use their respective coefficients for the ratio. This calculator handles the two substances you specify; for reactions with several reactants, also check the Limiting Reagent calculator.