GATE CE 2021 Set 1 — Question 43
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Environmental Engineering → Water Quality → MPN Test & Coliforms
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Question
A water sample is analyzed for coliform organisms by the multiple-tube fermentation method. The results of confirmed test are as follows:
The most probable number (MPN) of coliform organisms for the above results is to be obtained using the following MPN Index.
The MPN of coliform organisms per 100 mL is
| Sample size (mL) | Number of positive results out of 5 tubes | Number of negative results out of 5 tubes |
|---|---|---|
| 0.01 | 5 | 0 |
| 0.001 | 3 | 2 |
| 0.0001 | 1 | 4 |
| MPN Index for Various Combinations of Positive Results when Five Tubes used per Dilution of 10.0 mL, 1.0 mL and 0.1 mL | |
|---|---|
| Combination of positive tubes | MPN Index per 100 mL |
| 0 - 2 - 4 | 11 |
| 1 - 3 - 5 | 19 |
| 4 - 2 - 0 | 22 |
| 5 - 3 - 1 | 110 |
Correct answer
(B) 110000
Solution
1.Identify the combination of positive tubes from the test results: For sample sizes 0.01, 0.001, and 0.0001 mL, the number of positive tubes is 5, 3, and 1 respectively. So the combination is 5-3-1.
2.The provided MPN Index table is for standard dilutions of 10.0 mL, 1.0 mL, and 0.1 mL. For the combination 5-3-1, the MPN Index is 110 per 100 mL.
3.Calculate the actual MPN by adjusting for the dilution factor. The test used dilutions that are 1000 times smaller than the standard ones ().
Therefore, the correct option is B.Continue learning with Success Tracker
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