Chapter 6 · Chemical Composition
6.4Counting Molecules by the Gram
12 min · two checks
Predict
What is the mass of 0.250 mol of H₂O?
The idea
- Compute molar mass and convert between grams and moles.
- Chain grams → moles → molecules.
Molar mass is the formula mass written in grams per mole. Water is 18.02 g/mol. It converts mass and amount: grams ÷ molar mass = moles, and moles × molar mass = grams. This is the conversion you will use more than any other in the course. The number comes from the periodic table; the unit g/mol is what makes the factor legal.
To reach a particle count, do not multiply grams by Avogadro’s number. Go through moles. Map: g → mol → molecules. Inverting molar mass is the classic slip: dividing when you needed mass, or multiplying when you needed moles. The unit you want at the end tells you which way the factor points.
Keep these
- Molar mass (g/mol) = formula mass (u), numerically.
- mol = g / (g/mol). g = mol × (g/mol).
- A full count is grams → moles → particles.
Worked path
How many molecules are in 9.00 g of H₂O? Molar mass 18.02 g/mol.
Observe
Map: g → mol → molecules.