Simple solution preperation question

[iTsLikeAnEgG]

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Hi, I have just started looking over a friends pharmtech books and I had question as to how a simple solution preparation problem should be set up. I'm a computer science major by trade but like to dabble a bit on the side in various other areas. I understand that these questions are generally proportionality based and the math is very simple but the phrasing of questions is a bit confusing to me.

The problem goes as such:
Prepare a 8 mL of hydrocortisone solution with a concentration of 5 mg / mL from 100 mg / 2 mL.
1) How many mL of hydrocortisone are needed?
2) How many mL of dilutant are needed?

How would I go about solving this?

 

[chakapow]

Hi, I have just started looking over a friends pharmtech books and I had question as to how a simple solution preparation problem should be set up. I'm a computer science major by trade but like to dabble a bit on the side in various other areas. I understand that these questions are generally proportionality based and the math is very simple but the phrasing of questions is a bit confusing to me.

The problem goes as such:
Prepare a 8 mL of hydrocortisone solution with a concentration of 5 mg / mL from 100 mg / 2 mL.
1) How many mL of hydrocortisone are needed?
2) How many mL of dilutant are needed?

How would I go about solving this?

Required volume * Required Concentration = 40mg hydrocortisone
Given concentration / 40mg hydrocortisone =
required volume of hydrocortisone = 0.8mL
Final volume(8mL)- Volume of hydrocortisone (0.8mL)= Required volume of dilutant (7.2mL)

 

[iTsLikeAnEgG]

Thanks a lot! I managed to get the .8mL and didn't realize I had to subtract from 8mL to get 7.2mL.

 

[owlegrad]

Required volume * Required Concentration = 40mg hydrocortisone
Given concentration / 40mg hydrocortisone =
required volume of hydrocortisone = 0.8mL
Final volume(8mL)- Volume of hydrocortisone (0.8mL)= Required volume of dilutant (7.2mL)

Hate to step on your toes but here is another way to do the problem:

Prepare a 8 mL of hydrocortisone solution with a concentration of 5 mg / mL from 100 mg / 2 mL.
1) How many mL of hydrocortisone are needed?
2) How many mL of dilutant are needed?

8 ml hc solution (total mls of hc solution) * 5 mg hc/ml hc solution (desired concentration) * 2ml stock hc solution/100mg hc = 0.8 ml stock hc solution - check units, looks good to me

8 ml hc solution (final product) - 0.8 ml stock hc solution = 7.2 ml of diluent


Yikes I hope that is legible! Actually now that I look at it I think our approaches are the same, but I already typed it out so I am gonna post anyway.

 

[chakapow]

@owlegrad: Is volume additivity often assumed in pharmacy school?

 

[owlegrad]

@owlegrad: Is volume additivity often assumed in pharmacy school?

Excellent question! I guess so. I mean I think it is assumed when you say diluent that additivity is valid.

I can't think of an example where it was not. In dosage forms I remember learning about things like solubility, etc. but I don't recall any questions that involved anything particularly more complicated than the above problem. I am not done with PK yet, but I don't think it is going to involve having to know what can be added to what.