Stoichiometry problems ask how much reactant is needed or how much product can form in a chemical reaction. The reliable method is to balance the chemical equation, convert the given quantities to moles, use the mole ratios and convert to the units requested. Skipping the unit conversions is one of the easiest ways to lose track of a calculation.
Balance the chemical equation first
The coefficients in a balanced chemical equation express mole ratios. They do not directly express grams or molecules in the sample you have. Balance atoms of each element on both sides before using the coefficients in a calculation.
Do not change subscripts in chemical formulas to balance an equation; doing so changes the substance. Change coefficients and check that the atom count matches on both sides.
Convert the given mass into moles
If a problem gives mass, divide by the molar mass to obtain the amount in moles. If it gives solution concentration and volume, convert volume to litres where required and use the relevant concentration relationship. Keep units with each step until they cancel or convert into the requested unit.
Use the periodic table or molar masses provided by your course. Rounding too early can create avoidable discrepancies, so keep a few extra digits in intermediate calculations and round the final answer appropriately.
Use mole ratios from the balanced equation
The coefficients connect reactants and products. Multiply the known mole amount by the coefficient ratio to calculate the corresponding amount of another substance. Then convert the result into grams, particles, volume or concentration as the question requires.
If more than one reactant is given, calculate how much product each reactant could produce. The reactant that produces the smaller theoretical amount is the limiting reactant under the stated reaction model.
Calculate theoretical yield and percent yield
Theoretical yield is the maximum amount of product predicted by the balanced equation and the limiting reactant, assuming the reaction proceeds as modelled. Actual yield is the amount collected experimentally. Percent yield is actual yield divided by theoretical yield, multiplied by 100 percent.
A low percent yield can result from incomplete reaction, side reactions, product loss or measurement issues, but do not claim a particular cause unless the experiment provides evidence. In a pure calculation question, show the numerical method and distinguish theoretical from actual yield.
Moles = mass (g) / molar mass (g/mol)
Percent yield = (actual yield / theoretical yield) x 100%Check the answer and significant figures
A product amount should follow from the limiting reactant, and units should match the quantity requested. Check the magnitude: if a calculation gives more product than permitted by the limiting reactant, review the ratio and unit conversions.
Use the significant-figure rules specified in your laboratory or course. Include the balanced equation, calculation steps and a final statement with units so the result is understandable.
- Balance the equation.
- Convert the given quantities into moles.
- Use the coefficient ratio from the balanced equation.
- Determine the limiting reactant when multiple reactants are present.
- Convert to the requested units and check significant figures.
Understanding the concept is the first step.
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