I need some math help!

VegasCPhT wrote the following at 18:10 on 8/29/10;

I can not seem to find anything to help me in my books!Please HELP!
A inhalant aerosol contains 1/4% isoproterenol hydrochloride. If each depression of the valve delivers a 120 mcg dose of the drug, how many doses are contained in a 15 ml aerosol container?

moderator wrote the following reply at 20:02 on 8/29/10;

0.25% = 0.25g/100mL
Remember, percent means g per 100 mL when talking about dissolving solids in liquids.

1g = 1,000,000 mcg
therefore, you multiply 1g by 1 million to get mcg.

0.25g = 250,000mcg which means
250,000mcg/100mL = Xmcg/15mL
X = 37,500mcg in the 15mL container

37,500mcg/120mcg = 312.5
Therefore, there are 312 doses in the container.

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VegasCPhT wrote the following reply at 20:22 on 8/29/10;

I have a job lined up to start, interview and everything went great! Pharm D gave me a math packet to do with 50 questions and I am struggling on the last few. If you could help me with these it would be great! You explain it much better than my Mosby's book! Thank you again!

1.)What is the percentage of benzalkonium chloride in a solution made by mixing 350 ml of a 1% solution with 250 ml of a 0.33% solution?

2.)Two Ounces of an elixir containing 33% alcohol are diluted to a total volume of 250 ml. What would be the percentage of the resulting solution?

3.) Rx- Zinc Oxide 2.1g
Calamine 2.1g
Petrolatum qs 60g
The rx was prepared as written and then the prescriber wished to change the concentration of zinc oxide to 12% w/w How many grams of a 25% zinc oxide ointment could be added to change the strength to 12% w/w of zinc oxide?

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moderator wrote the following reply at 21:08 on 8/29/10;

Remember, percent is g/100mL if dissolving a solid in a liquid.

1g/100mL x 350mL = 3.5g
0.33g/100mL x 250 mL = 0.825g
3.5g + 0.825g = 4.325g

350mL + 250mL = 600mL

You multiply a decimal by 100 to get percent.

4.325g/600mL x 100 = 0.72%

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moderator wrote the following reply at 21:14 on 8/29/10;

Remember, only the weight oz = 28.35g, but the volume ounce = 30mL. An elixir is a liquid, so we're measuring volume. Therefore, 2 ounce = 2x30mL = 60mL

volA x concA = volB x concB
60mL x 33% = 250mL x concB
concB = 7.92%

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moderator wrote the following reply at 21:27 on 8/29/10;

2.1g
Calamine 2.1g
Petrolatum qs 60g
12/25

You're mixing 2 different concentration ingredients to get a 3rd different concentration product. Therefore, this is an alligation problem.

First, figure out the concentration of zinc oxide. "q.s." means that the total weight including all ingredients is 60g. In other words, add enough petrolatum to make the total 60g. You multiply a decimal by 100 to get percent.

2.1g/60g x 100 = 3.5%

3.5% 13
12%
25% 8.5

8.5g 25% + 13g 3.5% = 21.5g 12%
X + 60g = Y

X/60g = 8.5g/13g
X = 39g of 25% Zn Oxide

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VegasCPhT wrote the following reply at 22:23 on 8/29/10;

I have one last question, I know the procedure but not how to word it......How would you word this answer? Using an electronic balance, the least weighable quanity is 50mg. One of the quanities in part a is too small (30mg) to be weighed accurately and an aliquot must be made. Using lactose as a diluent, mathematically explain how this would be done?

I am soooo stumped as to how to word this!!!

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