Q.Molar mass.

Step 1. Molar mass is defined as the mass, in grams, of exactly one mole of a substance (element or compound).

Step 2. For a single element, the molar mass in g/mol is numerically identical to that element's atomic mass in u (e.g. carbon: 12.0 u atomic mass ⇒ 12.0 g/mol molar mass).

Step 3. For a substance made of molecules (or, for an ionic compound, formula units), the molar mass in g/mol is numerically identical to its molecular mass (or formula mass) in u (e.g. water: 18.0 u ⇒ 18.0 g/mol).

Define molar mass and note the numerical equality with atomic/molecular/formula mass in u, giving an example.

• Forgetting to mention the numerical equality between molar mass (in g/mol) and atomic/molecular mass (in u), which is the key practical fact that lets molar mass be looked up so easily.