MCQEasyJEE 2025Rate of Reaction

JEE Chemistry 2025 Question with Solution

For bacterial growth in a cell culture, growth law is very similar to the law of radioactive decay. Which of the following graphs is most suitable to represent bacterial colony growth? Where NN - Number of Bacteria at any time, N0N_0 - Initial number of Bacteria.

Four graph options of $$N/N_0$$ versus time are shown, including increasing, decreasing, linear decreasing, and saturating curves.
  • A

    Graph 1

  • B

    Graph 2

  • C

    Graph 3

  • D

    Graph 4

Answer

Correct answer:A

Step-by-step solution

Standard Method

Given: Bacterial growth is stated to follow a law similar in form to radioactive decay, but for growth the number increases with time.

Find: Which graph best represents bacterial colony growth.

For bacterial growth, the number of bacteria at time tt is given by

N(t)=N0ektN(t) = N_0 e^{kt}

where N0N_0 is the initial number of bacteria and kk is the growth constant.

So,

NN0=ekt\frac{N}{N_0} = e^{kt}

This means N/N0N/N_0 increases exponentially with time.

Therefore, the correct graph must show an upward-curving exponential increase as time increases.

The selected graph shows $$N/N_0$$ on the vertical axis and time on the horizontal axis, with an exponentially increasing curve.

Thus, the correct option is A.

Common mistakes

  • Choosing a decreasing curve by confusing bacterial growth with radioactive decay itself is incorrect. Radioactive decay decreases with time, but bacterial growth follows the exponential growth form. Use an increasing exponential graph instead.

  • Choosing the saturating curve is incorrect here because the given model assumes exponential growth. A leveling-off graph would require resource limitation, which is not part of the stated law.

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