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UPSC Combined Geo-Scientist

Chemist

  • 3 descriptive papers
  • 200 marks each
  • 3 h per paper

Mains Paper III covers analytical and organic chemistry.

UPSC Combined Geo-Scientist, Mains · 2021 · Mains Paper III

Q1(a). Q-test on four results for cadmium in dust

Question

The following values were obtained for the determination of cadmium in a sample of dust: 4.3, 4.1, 4.0 and 3.2 µg g⁻¹. Applying the Q-test, decide whether the last value, 3.2, should be rejected or not. [Given: Q critical for a sample size of four is 0.831]

Answer in brief

Qcalc = 0.8/1.1 = 0.727 is less than Qcrit = 0.831, so the value 3.2 is not rejected: it is retained.

Key points

  • Arrange in order: 3.2, 4.0, 4.1, 4.3; suspect value 3.2, nearest neighbour 4.0.
  • Gap a = 4.0 − 3.2 = 0.8; range w = 4.3 − 3.2 = 1.1; Qcalc = a/w = 0.727.
  • Qcalc (0.727) < Qcrit (0.831): 3.2 cannot be rejected at the 95 % confidence level.
  • As a doubtful value is kept, the median, 4.05 µg g⁻¹, is the safer figure to report.

The full model answer

As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.

Model answer to Q1(a), 2021 Mains Paper III: solution pages 1 and 2
Solution pages 1 and 2 of 39
Read the whole 2021 solution2021 question paper

Topic notes: errors in chemical analysis, standard deviation, confidence limits and the Q-, t- and F-tests, with every Civil Services, IFoS and Geo-Scientist PYQ →

CoversQ-testDixon Q testrejection of outlierstatistics of analytical data

All parts of Question 1