Neco 2017 Chemistry Question 42

Practice objective / multiple choice question 42 from the 2017 Neco Chemistry examination.

Neco 2017 Chemistry Objective / Multiple Choice Hard Difficulty
Question 42 NECO • 2017 • CHEMISTRY • objective

A wastewater sample contains an organic pollutant whose biochemical oxygen demand measured over 5 days (BOD\(_5\)) is \(45\ \text{mg L}^{-1}\). For this pollutant the chemical oxygen demand (COD) is approximately \(2.5\) times its BOD\(_5\). The water undergoes a two‑stage treatment: * Primary treatment removes \(20\%\) of the BOD and \(30\%\) of the COD. * Secondary biological treatment removes \(80\%\) of the BOD remaining after primary and reduces the COD by a further \(40\%\). Calculate (i) the final BOD\(_5\) concentration, (ii) the final COD concentration (to one decimal place), and (iii) the overall percentage removal of COD based on the initial COD.

Answer Options

A. BOD = 9.0 mg L⁻¹; COD = 31.5 mg L⁻¹; overall COD removal = 72 %
Correct Answer B. BOD = 7.2 mg L⁻¹; COD = 47.3 mg L⁻¹; overall COD removal = 58 %
C. BOD = 6.0 mg L⁻¹; COD = 45.0 mg L⁻¹; overall COD removal = 60 %
D. BOD = 7.2 mg L⁻¹; COD = 67.5 mg L⁻¹; overall COD removal = 40 %
Correct Answer
B
Correct Option:
BOD = 7.2 mg L⁻¹; COD = 47.3 mg L⁻¹; overall COD removal = 58 %

About This Question

This is Neco 2017 Chemistry Question 42. It is one of the objective questions from the 2017 Neco Chemistry examination.

Difficulty level: Hard .

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