Please Help Math Exam Retake Tomorrow Struggling

0Oct 20, '13 by Kat710Then set up the dimensional analysis so that "units" cancel and mL is left for the answer:
Please help.... I have asked and asked and cannot get a clear answer on how you "choose" the correct range of diluent in these kinds of questions. I assumed you pick the range closest to the order which would be 250,000, but the answer key says the correct choice is 200,000. I have to retake the math exam tomorrow to get into clinical and I am struggling with this type of question.... I am having the same problem with insulin and heprin... I cannot understand how to pick the right range of concentration for each question.... I am not having a problem with the DA math equations but with picking ranges... Please help... is there a RULE ?
Example
The order is to give 300,000 units of Penicillin G Potassium IM. A concentration of 200,000 units/ml should be used. The medication on hand is in a 1,000,000 unit vial. How many mL will you give?
Here are typical instructions for reconstitution of medication
supplied in a 1,000,000 unit vialConcentration Diluent to be Added 100,000 units/ml 200,000 units/ml 250,000 units/ml 9.6 ml4.6 ml3.6 ml According to these instructions, you should add 4.6 mL of dilutent to the stock penicillin.The reconstitution process.
Ans: 1.5 mLLast edit by Kat710 on Oct 20, '13 
1Oct 20, '13 by SlinkyheadRNWhy are you confused? The question tells you what concentration to use. There are 5mL in the vial. It says what concentration you have.KelRN215 likes this.

1Oct 20, '13 by Kat710Because this question is from a math help site, she is just stating what to use, but not saying why. Normally that part would not be included within the question. On my previous math exam there were several of these questions but "you" had to choose the correct dilution ratio... this is where I am lost. Even in my two nursing math books they work the problems out for you, but none of the books tell why or how they choose a particular dilution ratio....teacher08 likes this.

0Oct 20, '13 by Kat710I was told to use less diluent for IM injection for less total volume because the muscle can only hold so much and IV the larger range of diluent would be used... but if you look at this problem it is neither... it is right in the middle... I am very confused and wish someone could tell me the rule of thumb.

2Oct 20, '13 by SlinkyheadRNQuote from Kat710I believe 1.5 mL is okay for large muscles. But I may be wrong.I was told to use less diluent for IM injection for less total volume because the muscle can only hold so much and IV the larger range of diluent would be used... but if you look at this problem it is neither... it is right in the middle... I am very confused and wish someone could tell me the rule of thumb.
Also, there is no rule of thumb. You use the ratio that they tell you to use. You can't make up your own concentration. 
0Oct 21, '13 by teacher08Ok.....
Ordered: 300,000 units
On hand: 200,000 units/ml
Equation: 300,000 / 200,000 X 1 ml = 1.5 mL to give
The question ask how many mL to give. The instruction indicates a 4.6 mL diluent is to be used with a concentration of 200,000 units/mL. The diluent does not change the initial amount of the concentration. If I am incorrect, someone please correct me.
You are adding to the concentration not what was ordered. Some questions add additional information that may throw you off. Always look at what the question is truly asking you. It should be at the end of the question. Best wishes on your exam.
Also, I believe 1.5 mL is a safe amount for large muscles too.Last edit by teacher08 on Oct 21, '13 
0Oct 21, '13 by Kat710Thank you for the help and the best wishes. I appreciate it. I guess the question looks confusing because I posted the teachers guides in there.. The question normally would not have indicated 4.6ML... most of the questions we are getting on the tests ask "us" to pick the right diluent based on the order and it is never exact.
So the question would read like this:
Example
The order is to give 300,000 units of Penicillin G Potassium IM. The medication on hand is in a 1,000,000 unit vial. How many mL will you give?
Concentration Diluent to be Added 100,000 units/ml
200,000 units/ml
250,000 units/ml9.6 ml
4.6 ml
3.6 ml