Exit to H3 Spectroscopic Techniques (9813)

Structured Practice: UV/Visible Spectroscopy (H3)

Marked written questions on Beer–Lambert calculations, dilutions and calibration: convert transmittance to absorbance, spot an anomalous standard, stay inside the calibrated range and report the original concentration.

GCE A-Level H3 Chemistry 9813-2027

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Beer–Lambert analysis after a dilution. A stock solution of a dye is too concentrated to measure directly. A student pipettes 5.00 cm$^3$ of it into a 100.0 cm$^3$ volumetric flask and makes the solution up to the mark with solvent. In a 1.00 cm cell at the dye's $\lambda_{\max}$, the diluted solution transmits 35.5% of the incident light, so $I/I_0=0.355$. At this wavelength $\varepsilon=1.25\times10^{4}$ dm$^3$ mol$^{-1}$ cm$^{-1}$.

Part a: Calculate the absorbance of the diluted solution. Give three significant figures. (1 mark): Absorbance has no unit.
Part b: Calculate the concentration of the dye in the diluted solution. (1 mark): Include a unit such as mol dm-3 or µmol dm-3. Type a power of ten with e, as in 3.0e-5.
Part c: Calculate the concentration of the dye in the stock solution. (1 mark): Include a unit such as mol dm-3 or mmol dm-3.
Part d: Predict the absorbance the undiluted stock would give in the same cell at the same wavelength. (1 mark)
Part e: Another student reports the stock concentration as $1.80\times10^{-6}$ mol dm$^{-3}$ from the same data. Which mistake did they make? (1 mark)

Calibration of a coloured complex. A student makes a series of standards of a coloured complex, zeroes the spectrophotometer with a reagent blank, and measures each standard in the same 1.00 cm cuvette at the complex's $\lambda_{\max}$. The data are illustrative, not from a published analysis. Plot absorbance against concentration to answer the parts below.

Calibration standards measured at λmax in a 1.00 cm cuvette
Concentration / µmol dm⁻³	Absorbance
0.0 (reagent blank)	0.000
1.0	0.081
2.0	0.159
3.0	0.241
4.0	0.262
5.0	0.399

Part a: Give the concentration of the standard whose absorbance does not fit the trend of the others. (1 mark): Give the concentration in µmol dm-3.
Part b: Leave out that standard and find the gradient of the calibration line, in dm³ µmol⁻¹. Give the number only. (1 mark)
Part c: Sample P gives an absorbance of 0.212 under the same conditions. Find its concentration. (1 mark): Give the concentration in µmol dm-3.
Part d: Sample Q gives an absorbance of 0.74. What should the student do before reporting its concentration? (1 mark)
Part e: The student dilutes 2.00 cm³ of sample Q to 10.00 cm³, and the diluted solution gives an absorbance of 0.148. Find the concentration of the original sample Q. (1 mark): Give the concentration in µmol dm-3.

Structured Practice: UV/Visible Spectroscopy (H3)

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