Math Help......last 3

VegasCPhT wrote the following at 20:27 on 8/29/10;

1.) If 20g of petrolatum are added to 2 ounces of a 12.5% sulfur ointment, what is the percent strength of sulfur in the resulting ointment?

2.) You have on hand 800g of 5% coal tar ointment and 1200g of a 10% coal tar ointment.
a)if the two ointments are mixed, what is the concentration in the finished product? **8%**I was able to get this part! YA!
b)How many grams of coal tar should be added to the product to obtain an ointment containing 15% of coal tar?

3.) Rx- Aspirin 6 parts
Phenacetin 2 parts
Ceffeine 3 parts
Disp. 2 oz (av)
How many grams of each ingredient should be used to compound this Rx?

moderator wrote the following reply at 20:46 on 8/29/10;

There are 2 "ounce" in pharmacy, the apothecary ounce and the avoirdupois ounce. I am assuming that you mean avoirdupois ounce.

1oz (avoir) = 28.35g
Therefore, 2oz = 56.7g
56.7g ointment + 20g petrolatum ointment base = 76.7g ointment

Remember, percent is g/100g when dissolving solids in solids.
12.5g Sulfur/100g ointment = Xg/56.7g
X = 7.0875g Sulfur

Since you always multiply a decimal by 100 to get percent,
7.0875g Sulfur/76.7g ointment x 100 = 9.2%

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VegasCPhT wrote the following reply at 20:52 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 20:55 on 8/29/10;

When you mix 2 different concentration ingredients to get a 3rd different concentration product, that is an alligation problem. Remember, pure coal tar, is just 100% coal tar.

8% 85
15%
100% 7

85g 8% + 7g 100% = 92g 15%
2000g 8% + Xg 100% = Yg 15%

7g/85g = X/200g
X = 16.5g coal tar should be added to 2000g of the 8% coal tar to get 15% coal tar.

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moderator wrote the following reply at 20:59 on 8/29/10;

1oz (avoir) = 28.35g
2 oz = 56.7g
6 parts + 2 parts + 3 parts = 11 part total

56.7g/11 = 5.15g per part.

6x5.15 = 30.9g aspirin
2x5.15 = 10.3g phenacetin
3x5.15 = 15.45g caffeine

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