To prepare a mobile phase with a total volume of 42 mL and a ratio of 4:1:1 for 2-propanol, acetic acid, and water, we first need to determine the total number of parts in the ratio.
The ratio 4:1:1 means:
- 2-propanol: 4 parts
- Acetic acid: 1 part
- Water: 1 part
Total parts = 4 + 1 + 1 = 6 parts.
Next, we can find the volume of each component in the mobile phase by calculating the volume of one part:
\[ \text{Volume of one part} = \frac{\text{Total volume}}{\text{Total parts}} = \frac{42 , \text{mL}}{6} = 7 , \text{mL} \]
Now we can calculate the volume of 2-propanol, which is 4 parts:
\[ \text{Volume of 2-propanol} = 4 \times \text{Volume of one part} = 4 \times 7 , \text{mL} = 28 , \text{mL} \]
Therefore, the volume of 2-propanol that should be measured out to make 42 mL of mobile phase is 28 mL.