Open chemistry graphContribute

Before this: Mole ratios in reactions

Mass-to-mass stoichiometry

Balances measure grams, but equations work in moles. So a mass-to-mass problem always takes three steps.

  1. Grams to moles of the given substance, using its molar mass.
  2. Moles to moles, using the mole ratio from the balanced equation.
  3. Moles to grams of the wanted substance, using its molar mass.

grams A → moles A → moles B → grams B

Worked example: what mass of water forms when 8.00 g of H2 burns?

2 H2 + O2 → 2 H2O

Step 1: 8.00 g H2 ÷ 2.016 g/mol = 3.97 mol H2

Step 2: 3.97 mol H2 × (2 mol H2O / 2 mol H2) = 3.97 mol H2O

Step 3: 3.97 mol H2O × 18.015 g/mol = 71.5 g H2O

Common mistake

Using a mass ratio in place of a mole ratio. The coefficients compare moles, never grams. Going straight from grams of one substance to grams of another skips the step where the equation is actually used.

0 human reviews0 AI reviewsHelp review it

Watch Grams of H2 to grams of H2O
  1. For 2 H2 + O2 → 2 H2O, start with 8.00 g of hydrogen.
  2. Grams to moles, using the molar mass of H2: 3.97 mol.
  3. Moles to moles, using the equation: 2 mol of H2O for every 2 mol of H2, so 3.97 mol.
  4. Moles to grams, using the molar mass of H2O: 71.5 g.

0 human reviews0 AI reviews

Practice

1. 2 H2 + O2 → 2 H2O. What mass of water forms from 16.0 g of O2?

Answer recomputed0 human reviews0 AI reviews

2. 2 Mg + O2 → 2 MgO. What mass of MgO forms from 12.0 g of Mg?

Answer recomputed0 human reviews0 AI reviews

3. CaCO3 → CaO + CO2. What mass of CO2 forms from 50.0 g of CaCO3?

Answer recomputed0 human reviews0 AI reviews

4. N2 + 3 H2 → 2 NH3. What mass of H2 is needed to make 34.0 g of NH3?

Answer recomputed0 human reviews0 AI reviews