Molar Mass

Objective: Use atomic masses to compute simple relative formula masses.

Relative formula mass sums the appropriate atomic masses for each atom represented in a formula. Using approximate atomic masses H ≈ 1 and O ≈ 16, H₂O has relative molecular mass about 18; using Ca ≈ 40, Ca(OH)₂ has about 74. Molar mass has the same numerical value expressed in g/mol for ordinary isotopic composition, but relative formula mass itself is dimensionless. The periodic table’s atomic mass is an isotopic weighted average, so mass calculations are approximations at the stated precision. Percent composition by mass is component mass divided by total formula mass times 100%. Counts and parentheses must be applied before arithmetic.

Objective: Apply molar mass to calculations, evidence, and specified situations.

Molar mass is mass per mole of a substance. Multiply each element’s atomic mass by its formula count, including parentheses, then sum. The exercises explicitly give rounded atomic masses for reproducible arithmetic; a real calculation should use the selected periodic table and appropriate significant figures. Elemental mass contribution is count times atomic mass, so the most numerous element need not contribute the greatest mass. Distinguish g/mol from the mass of one molecule.