Chapter 6 · Chemical Composition
6.5Chemical Formulas as Conversion Factors
12 min · two checks
Predict
How many moles of H atoms are in 0.40 mol of H₂O?
The idea
- Use a subscript as a mole ratio.
- Convert grams of a compound to grams of an element inside it.
A formula is a set of equalities. In H₂O, 1 mol H₂O = 2 mol H = 1 mol O. In Ca(NO₃)₂, 1 mol compound = 2 mol N = 6 mol O. Those equalities are exact counts, so they do not limit significant figures. Combined with molar masses, they turn “grams of compound” into “grams of one element” through moles.
The map for “grams of O in 25.0 g of CaCO₃” is g CaCO₃ → mol CaCO₃ → mol O → g O. Skipping the mole in the middle and scaling by a mass fraction you invented is how this problem goes wrong. You may use mass percent instead, but mass percent itself comes from this same path.
Keep these
- Subscripts are mole-to-mole equalities.
- Convert through moles; do not jump from grams to grams by guess.
- Parentheses change the atom count and therefore the factor.
Worked path
How many grams of sodium are in 15.0 g of NaCl? Molar masses: NaCl 58.44 g/mol, Na 22.99 g/mol.
Observe
1 mol NaCl contains 1 mol Na.