The formula
grams → moles → equation ratio → moles → grams
What it means
A balanced equation says nothing about grams: it says how many molecules react with how many, and that means moles. So the route has three legs. First grams become moles by dividing by the molar mass; then the coefficient ratio is applied, which is where the chemistry lives; and finally moles become grams again by multiplying by the product's molar mass. Skipping the moles and doing a straight proportion with grams is the single most repeated mistake in the topic.
How to work it out by hand
- Work out the molar mass of the reactant and of the product from their formulas
- Divide the grams of reactant by its molar mass to get moles
- Multiply by the product's coefficient and divide by the reactant's
- Multiply the moles of product by its molar mass to get back to grams
What is worth knowing
This assumes the reactant you entered is the limiting one and that the reaction goes to completion. With two reactants in given amounts you first have to work out which runs out first, and if the reaction has an eighty per cent yield you multiply the answer by 0.8: the reaction yield calculator does that. The coefficients come from the balanced equation; if you do not have it yet, the equation balancer will give you the four numbers.