# Confusion with formula...

#### Zurfacej

Ok, I'm not sure if I'm breaking any rules by asking this... but I honestly can't figure out the formula for concentration/dilution. I'm currently not attending pharm tech school... trying to use mosby's PTCB exam review book to learn on my own (or until I can go to school).I just can't figure out this damn formula at all -.- please help!

initital volume (IV) x initial strength(IS) = final volume(FV) x final strength(FS)

Final Volume -(minus) Initial Volume equals amount of diluent to make final volume.

Question: How many milliliters of water should be added to 100 mL of 10% stock solution of sodium chloride to prepare a 0.9% solution of sodium chloride.

Answer: (10%)(100 mL) = (0.9%)(X mL), where X = 1,111 mL. To calculate the amount of diluent, subtract the initial volume from the final volume (1,111 mL-100 mL = 1,011 mL of diluent).

Where the heck are they getting 1,111 from, I honestly have no idea. This has me baffled. Are there any free online guides I could use to learn some of these formulas, because I am a bit lost with this one. I generally understand how most other formulas work, but this just has me stumped.

#### pharm B

##### Phar Noir
Lifetime Donor
10+ Year Member
The 1,111 mL is the X you solved for in the equation. You're given final strength and volume, along with initial strength. Re-arrange as necessary to isolate the X, giving you 1,111 mL.

(10/0.9)(100mL)= 1,111 mL

#### UCLA 2011

10+ Year Member
V = .1*100/(.009)

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#### Zurfacej

Ok... that makes sense, but honestly there's a few hundred examples in here and they all require different methods. Are there any ideas you can give me about how to go about finding answers to these?

Do I always isolate X (unknown/desired amount)

My reasoning behind this is that, in another problem I checked... I only got the answer 1 way... and it was totally different from the last method.

Question: The formula for a buffer solution contains 1.24% (w/v) of boric acid. How many milliliters of a 10% (w/v) boric acid solution should be used to obtain the boric acid needed in preparing 1 gallon of buffer solution.

Answer: 476 mL. 1 Gallon = 3,840 mL: This is a dilution problem, where IS = 10%, IV = unknown, FS = 1.24%, and FV = 3,840 mL: (10%)(IV) =(1.24%)(3,840), IV= 476 mL.

I did 3840*1.24(.1) and it turned out to 476.16, which is the correct equivilant. Did I do this correctly? or what? I can't figure out the order in which to do these steps. Any advice?

Like... what to do if I have (IS)(IV) = (FS)(x), I just don't know the orders for these at all =/

#### calisoca

10+ Year Member
I would have never thought to use SDN for answers to my homework problems. Friends? TA's? I suppose it's a decent resource though; whatever flips your pancake.

#### Zurfacej

I'm not trying to use SDN for "homework" lol. I'm actually preping for the PTCB using my own study materials: mainly working with Mosby's PTCB certification exam review book, and a calculations book. I just am having like serious problems with (v/w) (w/v) and dilution problems. I really don't know anyone in pharmacy so I can't just ask them... which really sucks =/

#### Hien

7+ Year Member
I suggest you go to cramster.com. They have many experts, ready to answer your question within 24hrs.

#### Kaiter

I'm not trying to use SDN for "homework" lol. I'm actually preping for the PTCB using my own study materials: mainly working with Mosby's PTCB certification exam review book, and a calculations book. I just am having like serious problems with (v/w) (w/v) and dilution problems. I really don't know anyone in pharmacy so I can't just ask them... which really sucks =/
He didn't really mean it in a rude way. Its just there are a lot of snakes on SDN who try to weasel their way around and get easy answers instead of trying to work on it on their own. I've seen a lot of it already and I've only been visiting here for a short time. Good luck with the studying though. I admire anyone who can study for anything on their own (even if an certification exam is 'easy')

#### siltears

Do cross multiplying that you learned in grade .....6?

old percent/old amount = new percent/new amount

make sure they are all in the same units, and it should be fine. at least in my experience

#### Deja

10+ Year Member
Ok... that makes sense, but honestly there's a few hundred examples in here and they all require different methods. Are there any ideas you can give me about how to go about finding answers to these?

Do I always isolate X (unknown/desired amount)

My reasoning behind this is that, in another problem I checked... I only got the answer 1 way... and it was totally different from the last method.

Question: The formula for a buffer solution contains 1.24% (w/v) of boric acid. How many milliliters of a 10% (w/v) boric acid solution should be used to obtain the boric acid needed in preparing 1 gallon of buffer solution.

Answer: 476 mL. 1 Gallon = 3,840 mL: This is a dilution problem, where IS = 10%, IV = unknown, FS = 1.24%, and FV = 3,840 mL: (10%)(IV) =(1.24%)(3,840), IV= 476 mL.

I did 3840*1.24(.1) and it turned out to 476.16, which is the correct equivilant. Did I do this correctly? or what? I can't figure out the order in which to do these steps. Any advice?

Like... what to do if I have (IS)(IV) = (FS)(x), I just don't know the orders for these at all =/
The two questions you asked are all asking for the samething, these problems are really easy, you just have to plug them in to the formula, I don't know why you used 3840*1.24*0.1.... but just think of it this way, you need 1 gallon buffer that contains 1.24% boric acid and you have 10%, so then just plug it into the formula... 10% is IS b/c that's what you are starting off with and 1 gallon of 1.24% is the final that you are trying to get... not that confusing, you learned these in elementary school, these problems just have a more complicated way of asking it i suppose

and when you have (IS)(IV) = (FS)(x), you (IS*IV)/(FS)

hope this helped and didn't confuse you even more lol

#### Charlemagne

The easiest way to remember this is:

M1V1=M2V2

where M1 equals initial molarity( or concentration) and V1 equals initial volume (in whatever measurement you're using). Consequentially, M2 equals the resultant concentration, and V2 equals the resultant volume.

Hope that helps.

Edit: ninja'd, sorry.

Last edited:

#### calisoca

10+ Year Member
The easiest way to remember this is:

M1V1=M2V2

where M1 equals initial molarity( or concentration) and V1 equals initial volume (in whatever measurement you're using). Consequentially, M2 equals the resultant concentration, and V2 equals the resultant volume.

Hope that helps.

Edit: ninja'd, sorry.
This is how I learned it. Short and sweet. The more docile side of biochemistry.

#### Charlemagne

This is how I learned it. Short and sweet. The more docile side of biochemistry.
Haha, yeah, you're right.

Imagine sitting in your chair during class and after going through this particular formula the professor says," Ok class, just a reminder, make sure you know the structure of all 21 of the amino acids by tomorrow. Ciao!"

*Hops into a Lamborghini Murcielago and drives off into the horizon*

Student 1 " Wha.... what just happened?

student 2 " Just ignore the plot hole and accept it."

#### calisoca

10+ Year Member
Haha, yeah, you're right.

Imagine sitting in your chair during class and after going through this particular formula the professor says," Ok class, just a reminder, make sure you know the structure of all 21 of the amino acids by tomorrow. Ciao!"

*Hops into a Lamborghini Murcielago and drives off into the horizon*

Student 1 " Wha.... what just happened?

student 2 " Just ignore the plot hole and accept it."

I spent many hours on my dry-erase board memorizing those suckers. Sadly I've forgotten most of them by now.

We had a pop quiz lab practical one day, I'll never forget. Two weeks into Biochem 1 we came in, at our seat was a vile of colored fluid, on the board were the instructions, "You have one hour to determine the composition of the fluid in front of you. Feel free to use any equipment in the lab, good luck." I've never busted out a Bradford assay so fast in my life. An hour later there were micropipette tips everywhere, buffers spilled all over the benches, people were sweating, and two girls were crying...phew.

#### Charlemagne

I spent many hours on my dry-erase board memorizing those suckers. Sadly I've forgotten most of them by now.

We had a pop quiz lab practical one day, I'll never forget. Two weeks into Biochem 1 we came in, at our seat was a vile of colored fluid, on the board were the instructions, "You have one hour to determine the composition of the fluid in front of you. Feel free to use any equipment in the lab, good luck." I've never busted out a Bradford assay so fast in my life. An hour later there were micropipette tips everywhere, buffers spilled all over the benches, people were sweating, and two girls were crying...phew.
Damn, that's rough. You forgot to mention Comassie dyes marking everyone's shirts blue. I can hardly wait.
( I have a couple of friends that have already taken Biochem, and the stories that come from them seem to correlate with this one.)

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