Wednesday, May 14, 2014

The True Purpose of Pharmacy Alligation

I've discovered the true purpose of alligation. 

This is a method for solving solution problems. Here's an example:

How many mL of water must be added to 300 mL of 70% alcohol solution to make a 40% alcohol solution?

So what is the true purpose of learning alligation? 

So you can help your kids when they have to do the same questions when they take Algebra. That's it. That's the ONLY reason.


Anonymous said...

And when you have to take the GRE

Hildy said...

I totally wish I had had you around when I was in school. Sadly, even after staring at the example for half an hour, I still don't understand how 4/3=300x transmutes into 4x=900. Sigh.

Anonymous said...

um,never seen this before....never learned in pharmacy school.Seems like lots of drawing and arrows. How about V1C1=V2C2? This does it for me pretty quickly.

(300mL)(70%) = (x mL)(40%)
x mL = (300mL)(70%)/40%
x mL = 525mL
(525-300mL) = 225mL

Anonymous said...

oh yeah, and if you do add 225mL water to 300mL alcohol solution, you probably won't really end up with 525mL of liquid. Adding water to alcohol doesn't really work that way!

Crazy RxMan said...


It is 4 / 3 = 300 / X

4 parts to 3 parts = 300 ml to X ml

So that's where it reduces to 4X = 900 or X = 225ml

Crazy RxMan said...


V1C1 = V2C2 works great, EXCEPT when you have a solution that has 0% active ingredient. For example, diluting a solution with H2O to reduce the strength. That's where Alligation comes in handy.

Anonymous said...

Crazy, V1C1=V2C2 still works. Got 40mL of 50% something and want to dilute it to 20%?

V1C1/C2 -->

(40mL)(50%)/20% = 100mL final volume. So 100mL-40mL = add 60mL diluent to get 100mL of 20%.

chemie said...

Crazy, did you really say that you sometimes dilute solutions with 0% active ingredient? Reducing the strength of something with 0% active ingredient sounds like the beginning of a joke about a homeopathic treatment.