Students should be able to: perform calculations for these titrations…

The idea

Every titration calculation follows the same three steps. Convert the known solution to moles with n = cV/1000, using the volume in cm³. Apply the mole ratio from the balanced equation; this is where most marks are lost, because 2HCl + Ba(OH)₂ → BaCl₂ + 2H₂O is a 2 : 1 ratio, not 1 : 1. Then divide the moles by the volume of the unknown solution in dm³ to get its concentration. If 25.0 cm³ of hydrochloric acid needs 22.40 cm³ of 0.100 mol dm⁻³ barium hydroxide, n(Ba(OH)₂) = 2.24 × 10⁻³ mol, so n(HCl) = 4.48 × 10⁻³ mol and c(HCl) = 0.179 mol dm⁻³. Use only concordant titres in the mean, and quote answers to three significant figures, because burette readings support that precision.

What earns the marks

• Converting a volume in cm³ to dm³ before multiplying by the concentration.
• Applying the mole ratio from the balanced equation, such as 2 : 1 for HCl with Ba(OH)₂.
• Dividing the moles by the volume of the unknown solution to obtain its concentration.
• The answer to three significant figures with the unit mol dm⁻³.
• Carrying an earlier incorrect value forward consistently, credited as error carried forward.

Common mistakes and how to put them right

• Dividing by 1000 twice, or not at all.

• Using a 1 : 1 ratio with a diprotic acid or with a Group 2 hydroxide.

• Including the rough titre in the mean before calculating.

• Quoting the answer to one significant figure when the burette readings justify three.

• Calculating the concentration of the solution in the burette when the question asks for the one in the flask.

Exam tips

• Write the balanced equation at the top of your working, even when the question does not ask for it; the ratio mark depends on it.
• Percentage uncertainty in a titre uses two readings, so double the burette uncertainty before dividing by the titre.