IB DP Chemistry -Structure 1.4 Counting particles by mass: The mole For SL Paper 1A -FA 2025
▶️ Answer/Explanation
Assume a 100 g sample. This gives:
C: 37.5 g / 12 g/mol = 3.125 mol
H: 12.5 g / 1 g/mol = 12.5 mol
O: 50.0 g / 16 g/mol = 3.125 mol
Dividing by the smallest number (3.125):
C: 3.125 / 3.125 = 1
H: 12.5 / 3.125 = 4
O: 3.125 / 3.125 = 1
Thus, the empirical formula is CH₄O.
✅ Answer: (D)
▶️ Answer/Explanation
Molar mass is defined as the mass of one mole of a substance.
That means its units must be “mass per mole”.
• The SI mass unit is \( \text{g} \) (grams) in this context.
• “Per mole” is written as \( \text{mol}^{-1} \).
Therefore, the correct unit is \( \text{g}\ \text{mol}^{-1} \).
✅ Answer: (D)
▶️ Answer/Explanation
A molecular formula tells the exact number of each atom present in one molecule of a substance.
• It does not show the simplest ratio of atoms—that is the empirical formula.
• It does not give bonding details.
• It does not give the number of molecules in a mole.
Therefore, the molecular formula provides the actual numbers of atoms in a molecule.
✅ Answer: (B)