# DBT JRF 2022 — Question 5

Exam: DBT JRF 2022 · Section: A · Topic: Biochemistry › Dilutions & Concentration Calculations · Difficulty: Medium

## Question

While measuring the potassium concentration in a drug formulation, the technologist took 10 µl of the sample and mixed it with 90 µl of test buffer. From the resulting solution, he took 5 µl and measured its total potassium content as 25 ng. What would be the concentration of potassium in the drug formulation?

## Options

- **A.** 50 ng/µl
- **B.** 25 ng/µl
- **C.** 75 ng/µl
- **D.** 100 ng/µl

## Answer

Option A

## Explanation

Serial dilution and concentration calculation: Step 1 — Dilution: 10 µL sample + 90 µL buffer = 100 µL total → dilution factor = 10 (i.e., 1:10 dilution). Step 2 — Concentration in diluted solution: 5 µL of diluted sample contains 25 ng, so: $\text{Conc. (diluted)} = \frac{25\,\text{ng}}{5\,\mu\text{L}} = 5\,\text{ng/}\mu\text{L}$ Step 3 — Back-calculate original concentration: $\text{Original conc.} = 5\,\text{ng/}\mu\text{L} \times 10 = 50\,\text{ng/}\mu\text{L}$ Answer: 50 ng/µL (Option 1).

Source: https://mypyqbuddy.com/questions/while-measuring-the-potassium-concentration-in-a-drug-6a104cd5920734408bc0c6e2