74. The volume (in ml) of a 1.0mg/mlstock solution of ampicillin to be added to 0.1 liter of growth
medium for achieving a final ampicillin concentration of 50𝜇 g/ml is ____
Ampicillin Stock Solution Calculation
Ampicillin dilution calculations ensure accurate antibiotic concentrations
in microbial growth media. To obtain a final concentration of
50 µg/ml ampicillin in 0.1 L medium from a
1.0 mg/ml stock solution, the required volume is
5 ml.
Correct Answer
Calculation Method
The standard dilution equation is:
where C1 = stock concentration, V1 = volume of stock
required, C2 = final concentration, and V2 = final
volume.
Unit Conversion
- Stock concentration (C1) = 1.0 mg/ml = 1000 µg/ml
- Final concentration (C2) = 50 µg/ml
- Final volume (V2) = 0.1 L = 100 ml
Solving the Equation
V1 = (50 × 100) / 1000 = 5 ml
Why 5 ml Works
Adding 5 ml of a 1000 µg/ml ampicillin stock provides a total of
5000 µg ampicillin. When added to ~100 ml medium, the
resulting concentration is approximately 50 µg/ml. In routine laboratory
practice, the small increase in total volume is neglected.
This solution matches the official answer for the
GATE XL 2015 microbiology question.
Common Errors Explained
- 1 ml mistake: Treats 1 mg/ml as 1 µg/ml and ignores unit
conversion. - 0.5 ml mistake: Uses 0.1 L directly instead of converting
to ml. - 50 ml mistake: Inverts the dilution formula, giving an
excessively high concentration.
Error Analysis Table
| Error Type | Calculation Flaw | Resulting Volume (ml) |
|---|---|---|
| No unit conversion | 50 × 100 / 1 | 5000 (impossible) |
| Volume mismatch | 50 × 0.1 / 1 | 0.005 |
| Formula inversion | 1000 × 100 / 50 | 2000 |
Lab Applications
Ampicillin stocks (commonly 50–100 mg/ml) are routinely diluted to
50–100 µg/ml for bacterial selection.
- Always sterile-filter stock solutions
- Store at −20°C
- Protect from light (light-sensitive antibiotic)
