4) Looking at the W/V formula, we know we need to reduce the fraction by half in this case in order to make our denominator go from 200ml - 100ml:

1.50g ÷ 2
200ml ÷ 2

5) Now, our converted fraction is:


6) Now look at the numerator ! Our answer is 0.75% W/V. Now that you have the (bottom) denominator at 100ml, whatever is in the (top) numerator is your % W/V.

-Don't let the decimals fool you. If you saw a 1 in the numerator, it would be 1%, right ?

-As you get better at this, you'll see that it's all about getting your fraction setup properly and then getting your denominator to 100ml.

That's really the basic rundown on Concentration Percentage as it's used in Pharmacy Tech Math. Whatever program you are studying will likely have a much more detailed approach and several practice problems.

The next step in learning about compounding math is learning about using
Alligations , which are used when mixing a preparation of a certain percentage when you have two different strengths to work with. Whatever you do, don't confuse the formulas we have done here with Alligations. You will know distinctly to use an Alligation when the question says you have two different strengths to work with.

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Go to Alligations Page

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Concentration Percentage Calculations

In Chemistry,
concentration percentages tell us how much of a particular active ingredient is in (or needs to be in) a total solution.

Initially, most pharmacy tech students who haven't taken advanced math or chemistry will struggle with how this works. So, please relax and don't worry if it doesn't click right away.

Many times you will see percentage concentrations
expressed as
1:100, 1:200, etc., which can be converted to a fraction by putting the first number on top of the second.

General Guidelines

1 ML
100 ML
Volume Percentage
1 GM
100 GM
Weight Percentage
1 GM
100 ML
Weight / Volume

1 part Solute (drug) of 100 parts Solvent (batch). There are not 101 parts, only 100. The solute is 1/100th of the solvent.

Next, notice you are
only working with Milliliters and Grams. If you can remember those factors you are most of the way there.

Let's jump into another example to help you better understand:

Unscented rubbing alcohol is generally sold as
70% isopropyl alcohol. That means it is 70% V/V. So, for every 100ml of solution, 70mL is isopropyl alcohol and the other 30mL is water. That also means that a liter of it has 700mL of isopropyl alcohol and 300mL of water.

Next we'll try this question below
and use a different method to solve it:

Example Question:

To compound a solution, 90ml of Drug "Q" is added to 0.6 Liters of water. What is the resulting percentage concentration of Drug "Q" ?

First, figure out what the Total volume is:
Drug "Q" =   90ml
Water = 600ml
Total Volume = 690ml

Next, take the
volume of Drug "Q" and divide it by the total volume.

90 ÷ 690 = .13**

Lastly, you can either move the decimal two places or
multiply by 100.

.13 → 13.
.13 X 100 = 13

Answer is: 13% V/V

**Side note - For the purpose of studying for the PTCB or ExCPT exams, round all decimals to the nearest hundredth. If your calculator says 0.130434 then go with .13

The example we did will work to determine what a Concentration Percentage is. However, what if we were asked the question this way:

Example Question:

How much Drug "Q" would be needed to compound 690mL of a 13% solution ?

First, take the V/V% Desired and divide it by 100.
(or move the decimal over two places).

13 ÷ 100 = .13

multiply the desired total volume by that.

690 X .13 = 90

Answer is: 90mL

We know the answer is correct by looking at the previous question and answer.

Question From Jennifer on Yahoo Answers:
What percent is a solution containing 150 mg of active substance in 20mL?

What you have is solution of 150mg in 20mL and you want to find out what percentage it is.

1) The first thing I like to do is get those milligrams turned into Grams since the W/V formula is 1g/100mL. We'll do that by multiplying our fraction by 10.

150mg x 10 = 1500mg (1.5g)
20ml     x 10 = 200mL

2)Next, convert the 1500mg to grams. Now your fraction is:


3)Now, remember the W/V formula:
Weight / Volume

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