Concentration and gas volumesIB MYP Chemistry: Revision notes
Section 1
Concentration of a solution
The concentration of a solution tells you how much solute is dissolved in a given volume of solution. It is measured in g dm⁻³ (g/dm³) or mol dm⁻³ (mol/dm³).
concentration (g dm⁻³) = mass of solute (g) ÷ volume (dm³)
Volumes are often given in cm³. Convert first: dm³ = cm³ ÷ 1000 (so 250 cm³ = 0.250 dm³).
Example: 4.5 g of salt in 500 cm³ is 4.5 ÷ 0.500 = 9.0 g dm⁻³.
Always convert cm³ to dm³ before using a concentration formula. Forgetting to divide by 1000 is the commonest error.
Section 2
Moles and concentration in mol dm⁻³
One mole of a substance has a mass equal to its relative formula mass in grams: moles = mass ÷ relative formula mass.
For solutions, concentration in mol dm⁻³ links moles and volume:
moles = concentration (mol dm⁻³) × volume (dm³)
Rearranged: concentration = moles ÷ volume.
Example: 4.5 g of NaCl (formula mass 58.5) is 4.5 ÷ 58.5 = 0.077 mol. In 0.500 dm³ the concentration is 0.077 ÷ 0.500 = 0.15 mol dm⁻³.
To change between units: g dm⁻³ = mol dm⁻³ × formula mass.
Use a triangle or write the formula out each time: moles = concentration × volume. Then rearrange for what you need.
Section 3
Molar volume of a gas
Equal numbers of moles of any gas occupy the same volume at the same temperature and pressure. At room temperature and pressure (rtp), one mole of any gas occupies 24 dm³.
volume of gas (dm³) = moles × 24
Example: 0.025 mol of hydrogen at rtp has a volume of 0.025 × 24 = 0.60 dm³ = 600 cm³ (1 dm³ = 1000 cm³).
Use the balanced equation to link moles of a reactant to moles of gas. For Mg + 2HCl → MgCl₂ + H₂, 1 mol of Mg gives 1 mol of H₂.
Section 4
Titration calculations (1 : 1 reactions)
In a titration, a solution of known concentration (from the burette) is added to a measured volume of another solution until the end point, shown by an indicator. The volume added is the titre.
Steps for a 1 : 1 reaction such as NaOH + HCl → NaCl + H₂O:
- Calculate moles of the solution you know: moles = concentration × volume (in dm³).
- Use the 1 : 1 ratio, so moles of the other solution are the same.
- Concentration = moles ÷ volume (in dm³).
Example: 20.0 cm³ of 0.100 mol dm⁻³ HCl neutralises 25.0 cm³ of NaOH. Moles HCl = 0.100 × 0.0200 = 0.00200 mol, so NaOH is 0.00200 mol and its concentration is 0.00200 ÷ 0.0250 = 0.0800 mol dm⁻³.
Must know
- concentration (g dm⁻³) = mass ÷ volume in dm³
- dm³ = cm³ ÷ 1000
- moles = mass ÷ formula mass
- moles = concentration (mol dm⁻³) × volume (dm³)
- one mole of any gas occupies 24 dm³ at rtp
- titration: find moles of the known solution, use the 1 : 1 ratio, then divide by the unknown volume
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Concentration and gas volumes
- A pharmacist in Lagos prepares a saline solution for a clinic by dissolving 4.5 g of sodium chloride in water and making the solution up to 500 cm³.The pharmacist needs 250 cm³ of this solution for a patient. Calculate the mass of sodium chloride in 250 cm³ of the solution.2 marks
- A student in Singapore reacts magnesium ribbon with excess dilute hydrochloric acid at room temperature and pressure. The equation is Mg + 2HCl → MgCl₂ + H₂. At room temperature and pressure, one mole of any gas occupies 24 dm³. The relative atomic mass of magnesium is 24.Calculate the volume of hydrogen, in cm³, produced when all of the 0.60 g of magnesium reacts.2 marks
- A quality-control chemist at a food factory in Seville checks the concentration of a sodium hydroxide cleaning solution by titration. She measures 25.0 cm³ of the cleaning solution into a conical flask and adds hydrochloric acid of concentration 0.100 mol dm⁻³ from a burette until the indicator changes colour. The mean titre is 20.0 cm³. The equation is NaOH + HCl → NaCl + H₂O.Calculate the number of moles of hydrochloric acid used in the titration.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).