How to Dilute Alcohol Solutions: A Practical Guide

Understanding how to dilute alcohol solutions, such as ethanol, is crucial in various fields, including pharmaceuticals, laboratories, and cleaning. This guide provides clear steps on how to achieve a desired alcohol concentration by adding water to an existing solution.

Suppose you have 8 liters of a 90% alcohol solution and you want to dilute it to a 40% alcohol solution. This article will walk you through the calculation process step-by-step.

Step-by-Step Calculation

The first step is to determine the amount of pure alcohol present in the initial solution. The formula is:

Pure alcohol (in liters) Volume (in liters) × Concentration

Step 1: Calculate the Pure Alcohol

In this case, the volume is 8 liters and the concentration is 90%, or 0.90 in decimal form. Therefore:

7.2 liters 8 liters × 0.90

This means that 7.2 liters of the 8 liters is pure alcohol.

Step 2: Set Up the Dilution Equation

The next step is to set up an equation that will help us find the volume of water to be added. Let's denote the volume of water to be added as ( x ) liters.

The total volume of the new solution will be ( 8 x ) liters. The final concentration is 40%, or 0.40 in decimal form. The equation for the final concentration is:

(frac{7.2}{8 x} 0.40)

Step 3: Solve for ( x )

Multiplying both sides by ( 8 x ) to clear the fraction gives:

7.2 0.40(8 x)

Expanding the right side:

7.2 3.2 0.4

Rearranging to isolate ( x ):

7.2 - 3.2 0.4

which simplifies to:

4.0 0.4

Dividing both sides by 0.40:

x frac{4.0}{0.40} 10) liters

Therefore, you should add 10 liters of water to achieve the desired 40% alcohol concentration in 8 liters of 90% alcohol.

Considerations and Practical Insights

It's important to be aware that when alcohol is mixed with water, the two compounds will mix to some extent, so volumes may not add up exactly. For example, adding 50 mL of ethanol to 50 mL of water will yield a solution that is slightly less than 100 mL. However, for the purpose of this calculation, we assume the volumes are additive.

In the case of the 8 liters of 90% alcohol, you can verify the total volume of the final solution:

The total amount of pure alcohol is 7.2 liters, which is 40% of the final solution. Therefore:

(frac{7.2}{V_{final}} 0.40)

Solving for ( V_{final} ):

(V_{final} frac{7.2}{0.40} 18) liters

Since you started with 8 liters, you must have added 10 liters of water.

It is also important to note that alcohol concentrations need to be at least 70% to effectively kill viruses like the one causing the COVID-19 pandemic. Hence, if you want to make a 70% solution from a 90% one, you would follow a similar process but with a different final concentration.

Conclusion

By following these clear, step-by-step calculations, you can successfully dilute alcohol solutions to desired concentrations. Whether you need a 40% solution for a lab experiment or a 70% solution for disinfection purposes, the principles outlined here will apply.

Key Takeaways

Calculate the amount of pure alcohol in the initial solution before dilution. Use the dilution equation to determine the volume of water needed to reach the desired concentration. Be aware of the non-ideal mixing behavior of alcohol and water.