If you have 24.0 g of a substance with a molar mass of 12.0 g/mol, how many moles do you have?

• A. 0.50 mol
• B. 2.00 mol
• C. 24.0 mol
• D. 12.0 mol

Answer: (B)

When you want to know how many moles you have from a mass, use the relation moles = mass ÷ molar mass. The molar mass tells you how many grams correspond to one mole, so dividing the mass by this value cancels the grams and leaves moles.

Here, 24.0 g divided by 12.0 g/mol equals 2.00 mol. The units cancel (g ÷ g/mol = mol), and the significant figures align with the given data, so the result is 2.00 moles.