Quantitative Chemistry
What's covered
Key facts
Adding water to a solution decreases its concentration.
Atoms are rearranged but never created or destroyed in a chemical reaction.
Equal volumes of different gases at the same temperature and pressure contain equal numbers of molecules (Avogadro's law).
Adding more of the excess reactant to a reaction does not increase the amount of product formed (it stays limited by the limiting reactant).
In 2H₂ + O₂ → 2H₂O, 2 mol of H₂ produces 2 mol of H₂O; 0.5 mol of H₂ produces 0.5 mol of H₂O.
In CaCO₃: Mr = 40 + 12 + 48 = 100; %Ca = 40/100 × 100 = 40%.
Relative formula mass (Mr) is found by adding the relative atomic masses of all atoms in the formula.
The Avogadro constant is approximately 6.02 × 10²³ (per mole).
Actual yield is almost always less than theoretical yield because: the reaction may be reversible, the reaction may be incomplete, side reactions may produce unwanted by-products, and some product is lost during separation/purification.
To convert volume in cm³ to dm³, divide by 1000.
Sample questions
A taste of the 122 questions in this topic — answers marked. Sign up to practise the full set with spaced repetition.
A solution contains 0.5 mol of NaCl dissolved in 250 cm³ of water. What is its concentration in mol/dm³?
- •0.125 mol/dm³
- •0.5 mol/dm³
- ✓2 mol/dm³
- •500 mol/dm³
A student heats magnesium in open air and finds the product weighs more than the original magnesium. Why?
- ✓Magnesium reacts with O₂; product MgO contains the extra oxygen mass
- •Mass is not conserved in combustion; some atoms convert to energy
- •Mg absorbs heat from the flame, and this heat has mass
- •N₂ from air also reacts with Mg to form magnesium nitride, adding mass
What volume of CO₂ (at RTP) is produced when 10 g of CaCO₃ decomposes? (CaCO₃ → CaO + CO₂; Mr CaCO₃ = 100)
- •0.24 dm³
- ✓2.4 dm³
- •24 dm³
- •240 dm³
In the reaction 2H₂ + O₂ → 2H₂O, if 4 mol of H₂ is mixed with 1 mol of O₂, which is the limiting reactant?
- •H₂ — there is more H₂ than O₂, so H₂ runs out first
- •Neither — the 4:1 ratio matches the 2:1 stoichiometry
- ✓O₂ — 4 mol H₂ needs 2 mol O₂, only 1 available
- •O₂ — it has higher molar mass and reacts more slowly
Calculate the number of moles in 44 g of CO₂. (Mr of CO₂ = 44)
- •0.5 mol — CO₂ has two oxygen atoms, so divide by 2
- ✓1 mol (n = m ÷ Mr = 44 ÷ 44)
- •22 mol — molar gas volume is 22 dm³
- •44 mol — mass in grams equals moles for any substance
How do you calculate the percentage by mass of an element in a compound?
- •(Moles of element ÷ total moles in compound) × 100
- •(Mr of compound ÷ Ar of element) × 100
- ✓(Total Ar of element in formula ÷ Mr of compound) × 100
- •Ar of element × number of atoms of that element in the formula
Try it for four weeks. Free.
One school. Unlimited classes. No card limit. No teacher limit. If your students aren't practising daily by the end of the trial, you owe us nothing.