How do I prepare 40% v/v from 95% ethanol? When we say 95% ethanol, do we usually mean 95% w/w ethanol unless otherwise stated?
Please show me the calculation steps to prepare 40% v/v from 95% w/w ethanol.
Thanks.
To prepare a solution with 40% v/v ethanol using 95% w/w ethanol, you'll need to perform the following steps:
1. First, let's clarify the difference between w/w (weight/weight) and v/v (volume/volume) percentages. When we say "95% ethanol," it typically refers to 95% v/v ethanol unless otherwise stated. This means that 95% of the solution's volume is ethanol.
2. However, to prepare a solution with a different concentration, you need to consider the weight of the solute (ethanol) relative to the total weight of the solution. This is known as w/w percentage.
3. Now, let's calculate the amount of 95% w/w ethanol required to prepare 100 mL of a 40% v/v ethanol solution:
- Since you want to prepare 100 mL of the final solution, the volume of ethanol needed is 40 mL (40% of 100 mL).
- Let's assume the density of 95% w/w ethanol is 0.816 g/mL. This means that 100 mL of 95% w/w ethanol solution weighs 81.6 grams (density x volume).
- To find the weight of ethanol in this solution, multiply the total weight by the ethanol concentration: 81.6 g x 0.95 = 77.52 g.
4. Now that we know the weight of ethanol required (77.52 g), we can calculate the volume of 95% w/w ethanol needed to obtain this weight:
- Divide the weight of ethanol by the density of 95% w/w ethanol: 77.52 g ÷ 0.816 g/mL = 95 mL.
- So, you would need 95 mL of 95% w/w ethanol to obtain 40 mL of ethanol in a final volume of 100 mL.
5. To prepare the solution, measure 95 mL of 95% w/w ethanol and dilute it with a suitable solvent (such as distilled water or another solvent of your choice) to a total volume of 100 mL.
Remember to always handle chemicals safely and be aware of any hazards or restrictions associated with the use of ethanol.