MDCAT Chemistry · Chemistry · Fundamental concepts of chemistry. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
A mole is the amount of substance that contains 6.02 × 1023 particles. This number is Avogadro's number, NA. The particles may be atoms, molecules, ions or formula units, so always check which particle the question is counting.
The mass of one mole is the molar mass: the relative atomic or molecular mass written in grams. One mole of any gas occupies 22.4 dm3 at STP. Moles therefore link mass, number of particles and gas volume.
The empirical formula is the simplest whole-number ratio of the atoms in a compound. The molecular formula is a whole-number multiple of it. Percentage composition gives the mass of each element in 100 g of the compound.
Students stop at the number of molecules when the question asks for atoms or ions. Multiply by the number of atoms (or ions) in one formula unit, for example 3 H atoms in each NH3.
Answer: C6H12O6
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