Mole Calculator From a Chemical Equation: Moles of Product From the Mole Ratio

This calculator uses a balanced chemical equation to turn an amount of one substance (A) into the amount of another (B). Enter the mass and molar mass of A, the coefficients of A and B in the balanced equation and the molar mass of B; you get the moles of B, the grams of B and the moles of A you started from.

Formula

n_A = m_A ÷ M_A; n_B = n_A × (b ÷ a); m_B = n_B × M_B.

• n_A, n_B: amounts of A and B in moles
• m_A, m_B: masses in grams
• M_A, M_B: molar masses in g/mol
• a, b: the coefficients of A and B in the balanced equation; b ÷ a is the mole ratio

Worked Example

Ammonia synthesis, N₂ + 3 H₂ → 2 NH₃: how much ammonia can 10 g of hydrogen give? M(H₂) = 2 × 1.008 = 2.016 g/mol; M(NH₃) = 14.007 + 3 × 1.008 = 17.031 g/mol.

1. Moles of H₂: 10 ÷ 2.016 = 4.9603 mol
2. Mole ratio NH₃ : H₂ = 2 : 3, so n(NH₃) = 4.96032 × 2 ÷ 3 = 3.3069 mol
3. Mass of NH₃: 3.30688 × 17.031 = 56.32 g

Assumptions

• The equation is balanced; the ratio b ÷ a is only valid for balanced coefficients.
• A is the limiting reactant, or at least there is enough of every other reactant; otherwise the real amount of B is smaller.
• The reaction goes to completion, so the result is a theoretical (maximum) amount.
• If you already know the moles of A, enter them as the mass and set the molar mass of A to 1.

How to read the result

The moles of B are what the equation predicts from your amount of A; the grams of B are the same amount on the balance. Because the result assumes complete reaction, compare it with what you actually isolate in the percent yield calculator. Molar masses come from the molar mass calculator.

Limits and common mistakes

• Using the coefficients of an unbalanced equation gives a wrong ratio.
• Swapping a and b inverts the ratio: the coefficient of the substance you start from goes in the denominator.
• For two reactants, run the calculation for each; the one giving less product is limiting.

Frequently Asked Questions

How do I find moles of product from a chemical equation?

Convert the reactant to moles, multiply by the coefficient of the product and divide by the coefficient of the reactant.

How much ammonia comes from 10 g of hydrogen?

About 3.3069 mol, or 56.32 g, if the hydrogen is fully used.

What is a mole ratio?

The ratio of the coefficients of two substances in a balanced equation; it is the same as the ratio of their amounts in moles.

in ga | Mass of substance A | g | 10 | 0
in ma | Molar mass of A | g/mol | 2.016 | 0.001
in a | Coefficient of A in the equation | - | 3 | 0.000001
in b | Coefficient of B in the equation | - | 2 | 0.000001
in mb | Molar mass of B | g/mol | 17.031 | 0.001
out nb | Amount of B | mol | ga / ma * b / a | 4
out gb | Mass of B | g | nb * mb | 2
out na | Amount of A | mol | ga / ma | 4