Chemistry / Unit 3 Chem 0474 worked example, 60 to 90 seconds
Stoichiometric Calculations: Mass to Mass
The idea
A mass-to-mass stoichiometric calculation converts a given mass of one substance to moles, applies the balanced equation's mole ratio to find moles of a second substance, then converts those moles back to a mass.
In practice
How many grams of water form from 10.0 g of hydrogen reacting completely with excess oxygen, given 2H2 + O2 → 2H2O? Dividing 10.0 g by hydrogen's molar mass, 2.016 g/mol, gives about 4.96 mol H2. The equation's 2-to-2 mole ratio between hydrogen and water means 4.96 mol H2 produces 4.96 mol H2O. Multiplying by water's molar mass, 18.02 g/mol, gives about 89.4 g of water formed.
Where people slip
Skipping the mole-ratio step and multiplying masses directly ignores that grams of reactant and grams of product relate through moles, never through mass alone.
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