Question

To make 100mL of 5mg/mL using a stock solution of 50mg/mL, how much diluent is needed?
A. 50mL diluent
B. 70mL diluent
C. 90mL diluent
D. 100mL diluent

Answer 1

To make a final volume of 100 mL of a 5 mg/mL solution from a stock solution of 50 mg/mL, we need to calculate how much of the stock solution and how much diluent to use.

1. **Calculate the total amount of solute needed**:
\[
\text{Total amount of solute} = \text{Desired concentration} \times \text{Final volume} = 5 \, \text{mg/mL} \times 100 \, \text{mL} = 500 \, \text{mg}
\]

2. **Calculate the volume of stock solution needed**:
\[
\text{Volume of stock solution} = \frac{\text{Total amount of solute}}{\text{Concentration of stock solution}} = \frac{500 \, \text{mg}}{50 \, \text{mg/mL}} = 10 \, \text{mL}
\]

3. **Calculate the volume of diluent needed**:
\[
\text{Volume of diluent} = \text{Final volume} - \text{Volume of stock solution} = 100 \, \text{mL} - 10 \, \text{mL} = 90 \, \text{mL}
\]

Therefore, the amount of diluent needed is **90 mL**.

The correct answer is: **C. 90 mL diluent**.