AeroLab Reading room

CHEM / AS level

Practical chemistry and analysis

Volumetric analysis requires clean apparatus, controlled endpoints and uncertainty estimates. Concordant titres support consistency but do not by themselves prove accuracy.

Learning objectives

The big idea

Volumetric analysis requires clean apparatus, controlled endpoints and uncertainty estimates. Concordant titres support consistency but do not by themselves prove accuracy.

Qualitative analysis and mass or infrared spectra constrain identity. Combine evidence with formulae and chemical tests; no single peak usually identifies every structural feature. Use supplied data instead of unsupervised chemical handling.

Equations and relationships

percentage uncertainty = absolute uncertainty / measurement × 100

Use an appropriate uncertainty model for the apparatus.

Worked example

A 25.0 cm³ result has absolute uncertainty 0.10 cm³. Find percentage uncertainty.

Follow the solution

0.10/25.0 ×100=0.40%.

Investigate and explain

Predict the worked result before calculating. Change one input, record the new result, and explain which assumptions still hold.

Use the evidence activity below; no exact PhET model is claimed.

Open the interactive investigation

Check your understanding

1. Why rinse a burette with its intended solution?

  1. To change the solution concentration
  2. To avoid dilution by residual water
  3. To remove all measurement uncertainty
Check the answer and reasoning

To avoid dilution by residual water

Residual water would dilute the solution; rinsing reduces that source of bias.

2. A 25.0 cm³ result has absolute uncertainty 0.10 cm³. Find percentage uncertainty.

  1. 4.0%
  2. 0.004%
  3. 0.40%
Check the answer and reasoning

0.40%

0.10/25.0 ×100=0.40%.

3. Does an IR absorption prove a unique complete structure?

  1. No
  2. Yes
  3. Only if strong
Check the answer and reasoning

No

Functional-group evidence must be combined with other measurements to distinguish possible structures.