# Per Cent concentration drug calculations



## 8jimi8 (Feb 20, 2011)

Doing my P2 homework has unearthed a weakness in my drug calculation prowess.


The question:  If you have a 4% solution of medication how many mls do you add to 500cc if you want a final concentration of 4mg/ ml


here's my rationale:  1% solution = 1 gram / 100 cc 

to arrive at a final concentration of 4 mg/ml i need a total of 2000mg over the 500 ml.

typing it out it seems pretty simple that the answer is 50cc, right?!


----------



## Sandog (Feb 21, 2011)

Is that all the info given? Seems like we need to know what molarity the solution is, in other words, what is the concentration of the solution, 4% of what?

How did you get 1%= 1 gram? 1 gram is quite a large amount of solute.

Maybe I am thinking too much like chemistry here, but something seems missing.


----------



## 8jimi8 (Feb 21, 2011)

Sandog said:


> Is that all the info given? Seems like we need to know what molarity the solution is, in other words, what is the concentration of the solution, 4% of what?
> 
> How did you get 1%= 1 gram? 1 gram is quite a large amount of solute.
> 
> Maybe I am thinking too much like chemistry here, but something seems missing.



That is a number i pulled off of the internet. we'll see if its right.

My instructions are " dosages must be perfect."


----------



## Sandog (Feb 21, 2011)

Lets examine this, you are saying 1% = 1 gram per 100cc or 1g/100ml since 1cc=1ml. 1 molar = 1 mole per liter and 1 mole = 1 gram so  at 1g/100ml means we have 10g at 1 liter or a 10 molar solution which seems like a real strong solution. In all fairness I am not a medic or nurse so I am not familiar with all the dosing issues but I do know gen chemistry and I keep thinking 4% of what? 

Since no one else spoke up I thought I would interject. Just trying to get the full picture as I am fairly good with stoichiometry...


----------



## 8jimi8 (Feb 21, 2011)

i dont know about your molar conversion,

from what I found on the internet the constant is 1 gram / 100 cc = 1% solution.

further reduced to 10mg / 1 ml.

MedicRob found some website backing up that number, so i'm gonna roll with it.

if that supposed number is correct, my calculations shouldn't be wrong.


the equation is actually pretty simple if you have the 1% solution figure memorized


----------



## Sandog (Feb 21, 2011)

I am certain my molar conversion is correct:
1 molar = 1 M = 1 mole/liter and moles = 1g/mol, 100cc = 100 ml so 1000ml = 1 liter thus if you have 1g/100cc then 1000cc(1 liter)/10g is 10 moles or a 10 molar solution.

My only assumption can be is that the 4% is a standard percentage that should be known otherwise I think the question is bogus. I am sure someone will set me straight so I am ready for mud in my face :sad:


----------



## Aidey (Feb 21, 2011)

While chemistry is not one of my strong areas, I think you are misapplying moles in this situation. 

1 gram = 1 mole of a pure substance. Since medications aren't pure substances 1g =/= 1 mole.

I think.


----------



## Sandog (Feb 21, 2011)

Aidey said:


> While chemistry is not one of my strong areas, I think you are misapplying moles in this situation.
> 
> 1 gram = 1 mole of a pure substance. Since medications aren't pure substances 1g =/= 1 mole.
> 
> I think.



Molarity = moles of solute/1 liter of solution

1 mole of a substance means there is 1 gram of substance, a 1 molar solution means there is 1g of substance dissolved in a 1 liter of solution eg. 
1g of poop/1L = 1 molar of poop solution.


----------



## 8jimi8 (Feb 21, 2011)

Sandog said:


> Molarity = moles of solute/1 liter of solution
> 
> 1 mole of a substance means there is 1 gram of substance, a 1 molar solution means there is 1g of substance dissolved in a 1 liter of solution eg.
> 1g of poop/1L = 1 molar of poop solution.





the misapplication being that the mole is 10x the amount of solvent we are talking about.

1% solution is equal to 1 gram in 100 Milliliters.


if a mole = 1gram / 1liter...


----------



## medicRob (Feb 21, 2011)

Percent solutions all are 1000mg/100cc.  

2% = 20mg/cc, 
5% = 50mg/cc, 
5.5% = 55mg/cc, 
etc...

4% = 40 mg/cc


----------



## Sandog (Feb 21, 2011)

8jimi8 said:


> the misapplication being that the mole is 10x the amount of solvent we are talking about.
> 
> 1% solution is equal to 1 gram in 100 Milliliters.
> 
> ...



Now I think I got myself confused... Arggg


----------



## Sandog (Feb 21, 2011)

medicRob said:


> Percent solutions all are 1000mg/100cc.
> 
> 2% = 20mg/cc,
> 5% = 50mg/cc,
> ...



Okay, I see, that was the magic number I was not aware of.

So that would imply a 1 molar solution? 100% = 1000mg = 1g yes?



Thanks ...


----------



## 8jimi8 (Feb 21, 2011)

Sandog said:


> Okay, I see, that was the magic number I was not aware of.
> 
> Thanks ...



It took him a long time to tell me the same thing.  I've been bugging him about that for like 6 hours.

apparently i'm to be "rickrolled" for even suggesting the theme tonight.


----------



## Aidey (Feb 21, 2011)

Stolen from Wikipedia's article on saline. 



> The solution is 9 grams of sodium chloride (NaCl) dissolved in 1 liter  of water. The mass of 1 milliliter of normal saline is 1.0046 grams at  22°C [3][4].  The molecular weight of sodium chloride is approximately 58 grams per  mole, so 58 grams of sodium chloride equals 1 mole. Since normal saline  contains 9 grams of NaCl, the concentration is 9 grams per liter divided  by 58 grams per mole, or 0.154 moles per liter.




If you post below applies to medications saline would have 9 moles per liter, which it doesn't. 



Sandog said:


> I am certain my molar conversion is correct:
> 1 molar = 1 M = 1 mole/liter and moles = 1g/mol, 100cc = 100 ml so 1000ml = 1 liter thus if you have 1g/100cc then 1000cc(1 liter)/10g is 10 moles or a 10 molar solution.
> 
> My only assumption can be is that the 4% is a standard percentage that should be known otherwise I think the question is bogus. I am sure someone will set me straight so I am ready for mud in my face :sad:


----------



## Sandog (Feb 21, 2011)

Yes, you are right, I see my chem has gotten rusty, I think I will get some sleep now, obviously I need it.


----------



## mgr22 (Feb 21, 2011)

8jimi8 said:


> Doing my P2 homework has unearthed a weakness in my drug calculation prowess.
> 
> 
> The question:  If you have a 4% solution of medication how many mls do you add to 500cc if you want a final concentration of 4mg/ ml
> ...



To go from a 4% solution (40mg/cc) to 4mg/cc (0.4%), you're decreasing the concentration by a factor of 10, which means you'd need 10 times the solvent (5,000cc). Of course, we'd never do that -- just as we'd never put 20,000mg (20g) of a med in a 500cc bag (well, maybe if Rev. Jim Jones were a nurse... . This sounds like an incredibly contrived problem, or someone made a mistake.


----------



## medicRob (Feb 21, 2011)

8jimi8 said:


> It took him a long time to tell me the same thing.  I've been bugging him about that for like 6 hours.
> 
> apparently i'm to be "rickrolled" for even suggesting the theme tonight.



In my defense, I was drinking... I was being attacked with dosage calculation during my drinking hour!


----------



## 8jimi8 (Feb 21, 2011)

medicRob said:


> In my defense, I was drinking... I was being attacked with dosage calculation during my drinking hour!



Fair enough, but you know i'll have to pay it forward to someone we treat at bonnaroo.


----------



## medicRob (Feb 21, 2011)

8jimi8 said:


> Fair enough, but you know i'll have to pay it forward to someone we treat at bonnaroo.



Get ready, cause it is gonna be crazy. You'll love it though.


----------



## 8jimi8 (Feb 21, 2011)

medicRob said:


> Get ready, cause it is gonna be crazy. You'll love it though.



I freaking tried to contact the Tennessee Board of Nursing. Those lame-os are closed for the holiday...


----------



## medicRob (Feb 21, 2011)

8jimi8 said:


> I freaking tried to contact the Tennessee Board of Nursing. Those lame-os are closed for the holiday...



Yeah, I saw your post on FB. They close for all sorts of weird pseudo-holidays


----------



## Sandog (Feb 22, 2011)

Aidey said:


> Stolen from Wikipedia's article on saline.
> 
> 
> 
> ...



For some dad gum reason I had 1 mole = 1 gram stuck in my head. This would only apply to hydrogen which is about 1 gram per mole. Now after looking at a periodic table I see how far off base I was as each element has a different mass per mole. Saline for example; Sodium Na = 22.98g/mol and 
Chloride Cl = 35.4g/mol so NaCl molar weight is ~58g/mol as you pointed out. 
Don't I feel silly. Embarrassing part is that I was so certain. Geez:blush:


----------



## 8jimi8 (Feb 22, 2011)

No regrets as long as learning occurs.


----------



## medicRob (Feb 22, 2011)

8jimi8 said:


> No regrets as long as learning occurs.



Second that. Next time you are drunk, I am gonna call you up and ask you questions about FiO2/SaO2 and ARDS.


----------

