5. Chemical Calculations HODDER
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Define a Mole.
The SI unit for amount of substance. It contains the same number of particles as there are atoms in 12g of Carbon-12.
What is the Avogadro Constant?
The number of particles in one mole: 6.02 × 10²³.
What is Molar Mass?
The mass of one mole of a substance in grams (numerically equal to the Mr or Ar).
What is the Molar Gas Volume at RTP?
24 dm³ (or 24,000 cm³).
State Avogadro’s Law.
At the same temperature and pressure, equal volumes of gases contain the same number of particles.
Equation: Calculate Moles from Mass.
Moles = Mass / Mr
Equation: Calculate Mass from Moles.
Mass = Moles × Mr
Equation: Calculate Moles of Gas (at RTP).
Moles = Volume (in dm³) / 24
How do you convert cm³ to dm³?
Divide by 1000.
What is Percentage Purity?
(Mass of pure substance / Total mass of impure sample) × 100.
What is Percentage Composition?
(Total Ar of element / Mr of compound) × 100.
What is the Limiting Reactant?
The reactant that is completely used up in a reaction. It determines the amount of product made.
What is the Reactant in Excess?
The reactant that is left over after the reaction stops.
Steps to calculate Empirical Formula?
• Mass (or %) ÷ Ar (find moles). 2. Divide all moles by the smallest number (find ratio). 3. Round to nearest whole number.
Difference between Empirical and Molecular Formula?
Empirical is the simplest ratio. Molecular is the actual number of atoms in the molecule.
Calculate the mass of 0.5 moles of Water (H₂O).
Mr of H₂O = 18. Mass = 0.5 × 18 = 9g.
How many atoms are in 1 mole of Oxygen gas (O₂)?
1.204 × 10²⁴ atoms. (1 mole of molecules = 6.02×10²³, multiply by 2 because O₂ is diatomic).
You calculate moles using V/24. The volume is 480 cm³. Is the answer 20 moles?
NO. You must convert to dm³ first. 480/1000 = 0.48 dm³. Moles = 0.48/24 = 0.02 mol.
Reaction: Mg + 2HCl -> MgCl₂ + H₂. If you have 1 mole of Mg and 1 mole of HCl, which is limiting?
HCl is limiting. The ratio is 1:2. You need 2 moles of HCl for every 1 mole of Mg, but you only have 1.
You have a concentration of 36.5 g/dm³ of HCl. What is this in mol/dm³? (Mr of HCl = 36.5).
1.0 mol/dm³. (Mass Conc / Mr = 36.5 / 36.5 = 1).