In short: If you rinse a burette with water and then fill it, the water left inside dilutes the solution. Its concentration goes down, so you need a bigger volume to react with the same amount in the conical flask. The titre goes up.
Practical error questions like this come up again and again in A Level Chemistry. They reward one habit: know the method, know why each step is there, and know what happens when it goes wrong.
The question
Dave rinsed his burette with water, filled it with sodium hydroxide solution and carried out a titration against hydrochloric acid in a conical flask. What happens to his mean titre?
What happens, step by step
- The water left in the burette mixes with the sodium hydroxide and dilutes it.
- So its concentration is lower: there are fewer OH⁻ ions in every cm³.
- The amount of acid in the conical flask has not changed, so the same number of moles of OH⁻ is still needed to neutralise it.
- So a bigger volume of the more dilute solution has to be added. The titre goes up.
What it does to the final answer
Say 25.00 cm³ of 0.100 mol dm⁻³ hydrochloric acid needs 25.00 cm³ of 0.100 mol dm⁻³ sodium hydroxide. If the water dilutes the sodium hydroxide by 5%, to 0.0950 mol dm⁻³, the titre rises to 26.32 cm³.
Dave still uses 0.100 mol dm⁻³ in his calculation, because that is what the label says. So he works out more moles of sodium hydroxide than really reacted, and his concentration of hydrochloric acid comes out at 0.1053 mol dm⁻³: about 5% too high.
Rinsing the equipment the right way
- Burette: rinse with deionised water to clean it, then rinse with a little of the solution it is going to hold, and fill it with that solution.
- Pipette: the same: rinse with the solution it is going to deliver. Water left in the pipette dilutes that solution, so fewer moles go into the flask and the titre goes down.
- Conical flask: rinse with deionised water only. Water in the flask does not matter, because it does not change the number of moles of the substance you pipette into it.
That contrast is a favourite exam question: water in the burette changes the titre, water in the conical flask does not.
Rich's tip: Ask what the water changes. In the burette or the pipette it dilutes a measured solution, so the titre changes. In the conical flask it changes nothing that matters.
Other titration errors to know
- Air bubble below the tap: if it escapes during the titration, the burette reading includes volume that was air, not solution. The titre is too high.
- Overshooting the end point: too much solution is added after the colour change. The titre is too high. That is why you do a rough titration first, then add the solution drop by drop near the end point.
- Reading the burette: read the bottom of the meniscus with your eye level with it, to two decimal places, ending in 0 or 5.
- Concordant titres: only average titres that agree closely, typically within 0.10 cm³ of each other. Leave the rough titre out of the mean.
Writing it in the exam
Give each link in the chain as its own sentence: the water dilutes the solution; its concentration falls; a bigger volume is needed to react with the same number of moles; so the titre increases. A one-word answer, "higher", does not explain anything.
If you want to practise practical questions like this, Dexter, the A Level Chemistry tutor I built, can take you through the practicals in your course and the errors that come up in exams.