H3 Chemistry 9813 · Study focus: H3 Chemistry: Use the Beer–Lambert law in concentration calculations
H3 Chemistry: Use the Beer–Lambert law in concentration calculations
Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.
Your success criteria
- Use the Beer–Lambert law in concentration calculations
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
Orientation and prerequisite retrieval
Beer–Lambert law is A=εcl.
At fixed ε and l, absorbance is proportional to concentration.
Definitions and exact language
At fixed ε and c, absorbance is proportional to path length.
Concentration is calculated as c=A/(εl).
With ε in mol⁻1 dm3 cm⁻1, path length should be in cm and concentration in mol dm⁻3.
Absorbance is dimensionless because the product εcl has cancelling units.
- Doubling concentration within the linear regime doubles absorbance.
- A complete calculation rearranges before substitution and reports the concentration unit.
Text alternative: Text alternative: Doubling concentration within the linear regime doubles absorbance. A complete calculation rearranges before substitution and reports the concentration unit.
Detailed molecular explanation
With ε in mol⁻1 dm3 cm⁻1, path length should be in cm and concentration in mol dm⁻3.
Absorbance is dimensionless because the product εcl has cancelling units.
Worked leaf-specific case
Doubling concentration within the linear regime doubles absorbance.
A blank corrects absorption from solvent and cuvette before analyte measurement.
- A candidate writes, “Beer–Lambert law is A=ε/(cl).” Which correction should replace it?
Open the feedback checkpoint after attempting
- Beer–Lambert law is A=εcl.
Guided evidence check
Measurements outside the linear range should be diluted and repeated.
The chosen wavelength commonly maximises analyte sensitivity while limiting interference.
- Which labelled diagram, spectrum or calculation would most directly disprove “Concentration is c=Aεl.”?
Open the feedback checkpoint after attempting
- Concentration is calculated as c=A/(εl).
Independent transfer
A negative concentration signals an invalid blank, calibration or calculation rather than physical analyte amount.
A complete calculation rearranges before substitution and reports the concentration unit.
- Design a specific chemical check that would expose the error in “Doubling concentration halves absorbance in the linear regime.”
Open the feedback checkpoint after attempting
- Doubling concentration within the linear regime doubles absorbance.
Misconception repair
Misconception: A calculated negative concentration is a meaningful negative amount of solute.
Repair: A negative concentration signals an invalid blank, calibration or calculation rather than physical analyte amount.
- Write leaf-specific feedback for the misconception “A calculated negative concentration is a meaningful negative amount of solute.”
Open the feedback checkpoint after attempting
- A negative concentration signals an invalid blank, calibration or calculation rather than physical analyte amount.
Practice, unseen assessment, re-test and next step
Complete the diagnostic questions, review the feedback, then attempt the final check.
After delayed re-test, continue to Plan quantitative analysis using UV/visible spectroscopy at /learning/h3-uv-visible-quantitative-analysis-lesson.html.
- Review the feedback for each diagnostic question, then attempt the final check.
Open the feedback checkpoint after attempting
- Before moving on, state this boundary: A complete calculation rearranges before substitution and reports the concentration unit.