Ointment Strength Calculation

by Tyson
(Canada)

I'm preparing for a PEBC EVALUATION exam and I'm stumped on this question. Several pharmacists and technicians have come up with the answer of 4.8g, however, the answer presented in the seminar indicates the answer to be 6g. Which answer is correct?

The question is :
How many grams of HC powder should be added to 32g of 5% HC oint to prepare an oint containing 20% of HC.

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Aug 15, 2011 Video reply:

Comments for
Ointment Strength Calculation

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Mar 15, 2012
 the answer
by: Anonymous

the answer is 4.8g, there are 1.6g of HC in the 32g 5% ointment, and there are 6.4g in a 32g 20% ointment,so all you have to do is (6.4g-1.6g= 4.8g)
Sep 06, 2011
 Easy Way to Allligate!!!
by: ChRxis

The answer is 6gm
-
100- - - - 15
-
- - -20
-
5- - - - - 80
-
Standard alligation, although you have no final amount. So in this case, all you need to do now is set up a proportion:

20 = x
80 15

if you consider the fact that 80 is representative of the 5% cream. 20 gm of the 5% cream is 80% of the entire amount. Therefore 6gm of HC powder is equal to the remainder.
Aug 31, 2011
 quantity x strength solution
by: Anonymous

The solution:
have/quantity x strength = need/quantity x strength.

have need
32g (5%) = ?g (20%)

divide by 20% on both sides

have need
32g (5%)/20% = ?g (20%)/20% (cancels out)

32 x 5 = 160
160 / 5 = 8g

Answer: 8 grams of HC
Aug 23, 2011
 6 grams
by: MTL

Determine HC powder is a 100% solution
Let x = grams of HC powder
Then:
100% of how much HC powder when added to 32 grams of 5% powder will make the 32 grams you have plus the grams you'll have to add of 20% ointment.

Mathematically it looks like this:

1x + 32*0.05 = (32 + x)*.2
1x + 1.6 = 6.4 + 0.2x
0.8x + 1.6 = 6.4
0.8x = 4.8
x = 6

This is straight Algebra and may be confusing to you, as I don't know what an alligation table is. If this doesn't make sense to you sorry if I confused you more.
Aug 17, 2011
 ?
by: Anonymous

32 grams * 5% = 20% * x x= unknown amount

160 = 20% * x

160/20= x

x= 8 grams
Aug 16, 2011
 previous problem
by: Anonymous

oops I spelled alligation incorrectly. sorry
Aug 15, 2011
 Same method for this problem?
by: Anonymous

That's good you posted another response. Earlier today I was trying to figure out why I couldn't get the same answer you got from the allegation table I made.

Can we use that same allegation method to solve this problem?

How much 75% ethanol is needed to make 300ml of 30% ethanol?
A. 80mL
B. 120mL
C. 180mL
D. 240mL
Aug 15, 2011
 Tricky prob ..we need help pls
by: Kim

I realize that I made a mistake working this problem (when I wrote it down). I used 50% instead of 5% . Don't want it to go without notice and confuse anyone.

I came up with 4.6 grams for final answer.

Tough one)
Aug 12, 2011
 My answer is 6 grams.
by: Kim

Use the standard alligation tic-tac-toe setup. Since you are not given the strength of the HC powder, make it 0% in the alligation.

50% 20% 2/5=.4 x 32g = 12.8 (13)
20%
0% 30% 3/5=.6 x 32g = 19.2 (19)


19 - 13 = 6g


Hope this helps.

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