Alcohol percentage

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Frontline Fighter

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This question has stumped my family of medical professionals!
70% alcohol is the ideal. For alcohol to work best as a disinfectant, it needs some water to create the necessary reaction. Higher percentage is not better, neither is lower. My problem: I have a 16 ounce bottles of 50% alcohol solution. I have a supply of 95% alcohol. How much of the 95% solution do I need to add to the 50% 16 oz. bottles to bring them up to a 70% solution? I’m sure the answer is glaringly obvious to the right mind, but it is frustrating people with advanced medical degrees!
Thank you
 
This is a classic problem for algebra classes! Go ask a 9th grader...

Suppose you have 16 oz. of 50% solution and x oz of 95% solution. How many ounces of the mixture are there? How many ounces of alcohol?

Now write an equation that says the concentration of the mixture is 70%, and solve for x.

There are other methods used by people who need to do this all the time; here is one place I recall seeing such a method: Mixing Milk and Butterfat (this contains two discussions of the algebra method for a similar problem, with the other near the end).

Essentially the same idea is a technique called "alligation" that is taught to some pharmacy students, and in other countries where older methods are still common.
 
Dr.Peterson said:
This is a classic problem for algebra classes! Go ask a 9th grader...

Suppose you have 16 oz. of 50% solution and x oz of 95% solution. How many ounces of the mixture are there? How many ounces of alcohol?

Now write an equation that says the concentration of the mixture is 70%, and solve for x.

There are other methods used by people who need to do this all the time; here is one place I recall seeing such a method: Mixing Milk and Butterfat (this contains two discussions of the algebra method for a similar problem, with the other near the end).

Essentially the same idea is a technique called "alligation" that is taught to some pharmacy students, and in other countries where older methods are still common.
Click to expand...
 
Frontline Fighter said:
This question has stumped my family of medical professionals!
70% alcohol is the ideal. For alcohol to work best as a disinfectant, it needs some water to create the necessary reaction. Higher percentage is not better, neither is lower. My problem: I have a 16 ounce bottles of 50% alcohol solution. I have a supply of 95% alcohol. How much of the 95% solution do I need to add to the 50% 16 oz. bottles to bring them up to a 70% solution? I’m sure the answer is glaringly obvious to the right mind, but it is frustrating people with advanced medical degrees!
Thank you
Click to expand...
You want some amount, S, of 70% alcohol. That will contain 0.7S alcohol.

You have 16 oz. of 50% alcohol. That will contain .5(16)= 8 oz of alcohol.
You have T oz of 95% alcohol. That will contain .95T oz of alcohol.

The combination will contain a total of T+ 16 oz of solution with 8+ .95T of alcohol. So you want T+ 16= S and 8+ .95T= .7S. Solve those equations for T.

We can replace the "S" in 8+ .95T= .7S with T+ 16: 8+ .95T= .7(T+ 16)= .7T+ 11.2.
Subtract .7T and 8 from both sides: .25T= 3.2. T= 3.2/.25= 4(3.2)= 12.8 oz of 95% alcohol.
 
I am concerned about MD's who have not completed 8th grade. I suspect that is a more pressing problem than getting alcohol to the most effective concentration.
 
Where have you met "MD's who have not completed 8th grade"? Oh, wait, Texas! Okay that makes sense! (I used to live in Mississippi where you often heard "Thank God for Texas. They make even us look good!")
 
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