Calculating from Experimental Results

Why this skill matters

Paper 6 regularly provides experimental data (masses, volumes, temperatures, titration readings) and asks you to perform calculations using them. These calculations bridge practical work and stoichiometry, and they carry significant marks. The ability to extract numbers from a results table and use them in formulae is a core examinable skill.

Method and technique

Moles from mass

Formula: n = m / Mr

Example: A student obtained 3.18 g of copper from the electrolysis of copper sulfate solution. Calculate the moles of copper obtained. (Ar of Cu = 63.5)

n = 3.18 / 63.5 = 0.0500 mol

Moles from solution data

Formula: n = c x V (where V is in dm3, so divide cm3 by 1000)

Example: 25.0 cm3 of 0.100 mol/dm3 sodium hydroxide solution. Calculate the moles of NaOH.

n = 0.100 x (25.0/1000) = 0.100 x 0.0250 = 0.00250 mol

Concentration from titration data

Steps:

1. Calculate moles of the substance you know (from concentration x volume).
2. Use the mole ratio from the balanced equation to find moles of the unknown.
3. Calculate concentration: c = n / V (in dm3).

Example: 25.0 cm3 of NaOH required 22.50 cm3 of 0.100 mol/dm3 HCl. Find the concentration of NaOH. (Reaction: NaOH + HCl goes to NaCl + H2O, ratio 1:1)

Moles HCl = 0.100 x 0.02250 = 0.00225 mol. Moles NaOH = 0.00225 mol (1:1 ratio). Concentration of NaOH = 0.00225 / 0.0250 = 0.0900 mol/dm3.

Percentage yield

Formula: % yield = (actual yield / theoretical yield) x 100

Calculate the theoretical yield from stoichiometry, then compare with the experimentally obtained mass.

Rate from data

Rate = change in measured quantity / time taken

For gas collection: rate = volume of gas (cm3) / time (s) For mass loss: rate = mass lost (g) / time (s)

Common errors and how to avoid them

Forgetting to convert cm3 to dm3: The concentration formula uses dm3. Divide cm3 by 1000. Using cm3 directly gives an answer 1000 times too large.

Using the wrong mole ratio: Always write out the balanced equation and read the ratio carefully. If the equation is 2NaOH + H2SO4, the ratio is 2:1, not 1:1.

Rounding too early: Keep full calculator values during intermediate steps and round only the final answer. Early rounding accumulates errors.

Wrong units in the answer: State the unit with your answer. Concentration in mol/dm3, mass in g, rate in cm3/s or g/s.

Calculating theoretical yield incorrectly: Use the moles of the limiting reagent, not the excess reagent, to calculate the theoretical yield.

Marking points examiners look for

• Correct formula stated or implied
• Correct substitution of values (with unit conversions)
• Correct mole ratio used from the balanced equation
• Answer to appropriate significant figures / decimal places
• Correct unit stated with the final answer
• Working shown clearly (method marks are available even if the final answer is wrong)