ZhixuChemistry | Connecting mass and moles
Experiments often measure mass, while chemical equations describe ratios between particles. Amount of substance connects the two. Specify the entities, convert using molar mass, then use a balanced equation.
Specify what one mole counts
Amount of substance is denoted n and measured in mol. One mole contains 6.02214076 × 10²³ specified elementary entities. These may be atoms, molecules, ions or formula units. For NaCl, use formula units rather than assuming the ionic crystal consists of separate NaCl molecules.
Convert mass using n = m/M
Here m is mass and M is molar mass. With m in g and M in g/mol, n is in mol. Using approximately 58.44 g/mol for NaCl, 5.844 g corresponds to 5.844/58.44 = 0.1000 mol. The molar mass depends on composition and the precision of the atomic masses used.
Balance first, then use the ratio
The equation 2H₂ + O₂ → 2H₂O gives an amount ratio of 2:1:2, not a mass ratio. If 0.20 mol H₂ reacts completely with excess oxygen, the theoretical yield is 0.20 mol H₂O. Using approximately 18.02 g/mol for water gives about 3.6 g, rounded to suit the input precision.
Identify the limiting reactant
With 0.20 mol H₂ and 0.05 mol O₂, oxygen can react with only 0.10 mol H₂. Oxygen is limiting, so the theoretical yield is 0.10 mol H₂O and 0.10 mol H₂ remains. Comparing each amount divided by its stoichiometric coefficient helps identify the limiting reactant.
Separate theoretical and actual yield
These calculations assume completion of the stated reaction without additional reactions. Real experiments may involve side reactions, losses or incomplete conversion. Check purity, yield and significant-figure conditions, and avoid confusing mass, amount and particle count.
Try it yourself
Using a NaCl molar mass of 58.44 g/mol, how many moles are in 11.688 g? What entities are being counted?
Show reasoning and answer
n = 11.688/58.44 = 0.2000 mol of NaCl formula units. The units g divided by g/mol give mol, providing a useful check on the conversion.
Use the method in your next study session
Write the chain: known mass → amount → reaction ratio → target amount → target mass. State the relationship and units at each step.
These guides explain general subject foundations for learning and revision. They are not official examination syllabuses or questions. Refer to the relevant official institutions for examination requirements.