Percent Yield Calculator: Actual Yield ÷ Theoretical Yield × 100

This calculator finds the percent yield of a reaction. Enter the mass of product you actually obtained, the moles of the limiting reactant, the coefficients of the limiting reactant and the product in the balanced equation, and the molar mass of the product; you get the percent yield and the theoretical yield in grams and moles.

Formula

Percent yield = actual yield ÷ theoretical yield × 100, where theoretical yield = n_lim × (b ÷ a) × M.

• Actual yield: mass of product isolated, in grams
• n_lim: moles of the limiting reactant
• a, b: coefficients of the limiting reactant and of the product in the balanced equation
• M: molar mass of the product in g/mol

If you already know the theoretical yield in grams, set n_lim to that mass, a and b to 1 and M to 1.

Worked Example

Burning magnesium, 2 Mg + O₂ → 2 MgO, with 0.2 mol of Mg as the limiting reactant; 7.25 g of MgO is collected. M(MgO) = 24.305 + 15.999 = 40.304 g/mol.

1. Theoretical moles of MgO: 0.2 × 2 ÷ 2 = 0.2000 mol
2. Theoretical yield: 0.2 × 40.304 = 8.061 g
3. Percent yield: 7.25 ÷ 8.0608 × 100 = 89.94 %

Assumptions

• The limiting reactant is known; if not, work out which reactant gives the least product first.
• The product is weighed pure and dry; solvent or water left in it pushes the yield up.
• The molar mass is that of the product as weighed (including water of crystallisation if it is a hydrate).

How to read the result

Most real reactions give less than 100%: some material stays in the flask or on the filter, side reactions use up reactant and reactions may not finish. A result above 100% means the product is impure or wet, or the theoretical yield was worked out wrongly; it never means extra product was made. For the moles of the limiting reactant, use the mass to mole calculator.

Limits and common mistakes

• Basing the theoretical yield on the reactant in excess instead of the limiting one gives too low a percentage.
• Mixing units: the actual and theoretical yields must both be in grams (or both in moles).
• Forgetting the mole ratio when the coefficients differ, for example 2 KClO₃ → 2 KCl + 3 O₂.

Frequently Asked Questions

What is the formula for percent yield?

Percent yield = actual yield ÷ theoretical yield × 100.

What is the percent yield if 7.25 g of MgO forms from 0.2 mol of Mg?

The theoretical yield is 8.061 g, so the percent yield is about 89.94%.

Can percent yield be over 100%?

Only when the product is impure or still wet, or when the theoretical yield was calculated wrongly.

What is the difference between theoretical and actual yield?

The theoretical yield is calculated from the limiting reactant assuming complete reaction; the actual yield is what you measure.

in act | Actual yield (mass obtained) | g | 7.25 | 0
in nl | Limiting reactant | mol | 0.2 | 0.000001
in a | Coefficient of the limiting reactant | - | 2 | 0.000001
in b | Coefficient of the product | - | 2 | 0.000001
in mp | Molar mass of the product | g/mol | 40.304 | 0.001
out py | Percent yield | % | act / (nl * b / a * mp) * 100 | 2
out tg | Theoretical yield | g | nl * b / a * mp | 3
out tm | Theoretical yield in moles | mol | nl * b / a | 4