All subjects · Chemistry · WCH11 (Unit 1)

Reacting masses, yield and atom economy

A balanced equation tells you how much product to expect. Percentage yield compares what you made with that, and atom economy tells you how much of the reactants ends up in the product you want.

Reacting masses

1. Moles of the substance you know: n = m / M.

2. Use the equation's ratio to get moles of the substance you want.

3. Convert back: m = n x M.

If two reactants are given, find which runs out first: the limiting reagent decides the amount of product.

Percentage yield

% yield = (actual mass obtained / theoretical mass) x 100. Yields are below 100% because the reaction may be incomplete or reversible, side reactions happen, and product is lost when transferring, filtering or purifying.

Atom economy

% atom economy = (molar mass of desired product / total molar mass of all reactants) x 100, using the balanced equation. Addition reactions have 100% atom economy. A high atom economy means less waste and is more sustainable, which matters even when the yield is high.

Worked example

What mass of calcium oxide forms when 25.0 g of calcium carbonate is heated? CaCO3 -> CaO + CO2 (Ca = 40.1, C = 12.0, O = 16.0)

  1. M(CaCO3) = 100.1; n = 25.0 / 100.1 = 0.250 mol
  2. Ratio 1 : 1, so 0.250 mol CaO
  3. M(CaO) = 56.1; m = 0.250 x 56.1

Answer: 14.0 g of CaO

Key idea: n = m/M, use the mole ratio, m = n x M. Yield compares actual with theoretical; atom economy compares useful product with all reactants.

Check yourself

  1. What mass of MgO forms when 4.86 g of Mg burns completely? 2Mg + O2 -> 2MgO (Mg = 24.3, O = 16.0)
    Show answer

    8.06 g. n(Mg) = 4.86 / 24.3 = 0.200 mol, so 0.200 mol MgO; 0.200 x 40.3 = 8.06 g.

  2. Heating 25.0 g of CaCO3 should give 14.0 g of CaO (to 3 s.f.), but a student obtains 12.0 g. Find the percentage yield, using the unrounded theoretical mass. (Ca = 40.1, C = 12.0, O = 16.0)
    Show answer

    85.6 %. Theoretical = 25.0 / 100.1 x 56.1 = 14.01 g; 12.0 / 14.01 x 100 = 85.6%.

  3. Find the atom economy for making CaO by CaCO3 -> CaO + CO2. (Ca = 40.1, C = 12.0, O = 16.0)
    Show answer

    56.0 %. 56.1 / 100.1 x 100 = 56.0%.

  4. Which type of reaction always has 100% atom economy?
    • Substitution
    • Addition
    • Elimination
    • Thermal decomposition
    Show answer

    Addition. In an addition reaction every reactant atom ends up in the single product.

  5. 0.200 mol of Mg is added to 0.300 mol of HCl. Mg + 2HCl -> MgCl2 + H2. Which is the limiting reagent?
    • Mg
    • Neither, they react exactly
    • H2
    • HCl
    Show answer

    HCl. 0.200 mol Mg would need 0.400 mol HCl; only 0.300 mol is present, so HCl runs out first.

  6. Which is NOT a reason for a percentage yield below 100%?
    • Some product is lost during filtration
    • The reaction has a high atom economy
    • The reaction is reversible
    • Side reactions form other products
    Show answer

    The reaction has a high atom economy. Atom economy is a property of the equation and does not cause product to be lost.

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Last updated 7 November 2026. Spotted a mistake? Tell us.