Chemistry · General chemistry · Concept

Stoichiometry and theoretical yield

Use a balanced reaction and the available reactants to predict how much product can form.

Start with the reaction, then use the quantities

Stoichiometry connects amounts of reactants and products through a balanced chemical equation. Before calculating a yield, establish which reaction you are modeling and check that atoms are conserved. For pure calcium carbonate reacting with aqueous hydrochloric acid, we use the reaction below.

C a C O_{3} (s) + 2 H C l (a q) \to C a C l_{2} (a q) + H_{2} O (l) + C O_{2} (g)

Check atoms in the balanced reaction
Element	Reactant side	Product side
Ca	1	1
C	1	1
O	3	3
H	2	2
Cl	2	2

Read the coefficients as mole ratios

The coefficient 2 before HCl means that one mole of CaCO₃ requires two moles of HCl. The understood coefficient 1 before CO₂ means one mole of CaCO₃ can produce one mole of CO₂. These are ratios of amounts in moles, not ratios of masses in grams.

\frac{2 mol HCl}{1 mol {CaCO}_{3}}, \frac{1 mol {CO}_{2}}{1 mol {CaCO}_{3}}

Why must we balance before calculating?

Changing a coefficient changes how many formula units take part. Changing a subscript changes the substance itself. Balance with coefficients while keeping CaCO₃, HCl, CaCl₂, H₂O and CO₂ chemically unchanged. The coefficients then provide the conversion ratios.

Use molar mass to move between grams and moles

A balanced equation relates particle counts and mole amounts. If an amount is given in grams, divide by molar mass first. Add the mass contribution of every atom in the formula: CaCO₃ has one calcium, one carbon and three oxygens.

M ({CaCO}_{3}) = 40.08 + 12.01 + 3 (16.00) = 100.09 g/mol

Find which reactant runs out first

Compare each available mole amount with the amount its coefficient requires. Dividing moles by the coefficient puts the reactants on a common basis. The smallest value limits the reaction. A reactant with the larger mass can still be the limiting reactant.

reaction extent = min (\frac{n ({CaCO}_{3})}{1}, \frac{n (HCl)}{2})

Convert the allowed product amount into the requested unit

Once the limiting amount is known, use the product coefficient to find moles of product. Multiply by the product’s molar mass to obtain grams. Carry units through the factors so you can see each connection.

m ({CO}_{2}) = n ({CO}_{2}) M ({CO}_{2})

Explain what theoretical yield means

Theoretical yield is the maximum predicted by the stated reaction and available reactants. It assumes complete conversion with no competing reaction. Actual recovered product may be lower because of incomplete reaction or collection losses. A chalk sample can also contain substances other than CaCO₃; this example explicitly assumes pure calcium carbonate.

What if actual yield is measured?

Percent yield compares a measured actual yield with the theoretical yield for the same product. Do not substitute a theoretical prediction for an experimental measurement.

percent yield = \frac{actual yield}{theoretical yield} \times 100 %

Common mistakes

Using gram ratios directly from reaction coefficients.
Assuming a reactant is in excess without checking its amount.
Reporting theoretical yield as if it were an experimental result.

Work through an example

Model 5.00 g of pure CaCO₃ with 0.200 mol HCl. What is the theoretical mass of CO₂?

Predict CO₂ from calcium carbonate →

Sources and scope

Authored study material. Tool results depend on the stated inputs and model assumptions.