pharmacy math / finding volume?

by Leah
(Stanfield, OR)

Cefaclor is a cephalosporin antibiotic used to treat bacterial infections. A physician has ordered Cefaclor 450mg tid. The drug is available as a 7.5% suspension. What volume should be given for each dose?
A. 15mL
B. 12mL
C. 8mL
D. 6mL

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pharmacy math / finding volume?

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Sep 23, 2011
by: Colin Fernandes

Simple calculation ( Use this standard formula with sums on percentages- Don't ask why)

7.5%= 7.5gms in 100
= 7500mgs in 100mls = 7500/100= 75mgs per ml

2nd stage: 450mgs/75mgs= 6mls

Sep 06, 2011
Math tips
by: ChRxis

You have to understand that percent means per 100.

W/V % = gm per 100ml. So a 5% solution (or suspension) means 5gm of active ingredient per 100ml of volume.

W/W % = gm (of active ingredient) per 100 gm of weight. Same here...5% cream is 5gm of active ingredient per 100 gm of cream base.

Now, if the patient needs 450mg/dose, you have to figure out how much of your 7.5% cefaclor suspension to use for the dose.

7.5% means 7.5 gm of active ingredient per 100 ml of volume. OR, 7500 mg per 100 ml, OR 75mg per 1ml.

Using a proportion:

75mg 450mg
----- = -------
1ml Xml

Jul 23, 2011
Where did the 100ml come from?
by: Anonymous

A suspension is a liquid. Percentage is 100. That is why ml is being used. That is the unit of measurement for liquids. It means that in every 100% of suspension, 7.5% of that 100% is the drug.That is where the 100ml came from.

Jul 21, 2011
by: Tom

It's a weight to volume ratio. When you see a percentage, such as 7.5%, it means 7.5/100. This is a weight/volume ratio.

For every 100ml of solution, there is 7.5g of that drug. When we do proportions math we need to match the units (mg to mg; ml to ml). So you convert g to mg and solve for x.

Jul 18, 2011
by: Anonymous

where did the 100ml come from?

Jul 15, 2011
The answer
by: Anonymous

Another way of writing 7.5% suspension is 7.5g/100ml. Convert 7.5g to 7500mg and then use this equation 7500mg/100ml=450mg/xml to solve x which gives 6ml. We can ignore tid since we are looking for the volume of one dose. (Answer:D)

Jul 12, 2011
by: Anonymous

I don't get it either, I'm taking my test August 9. why are we changing it into grams? someone help please.

Jul 09, 2011
by: Anonymous

I don't understad where the 100ml comes from?

Jun 23, 2011
by: Anonymous

100ml/7.5g * 0.45g = 6ml
Ans: D

Jun 01, 2011
finding volume
by: Anonymous

convert 450 mg to to 0.45 grams Then .045/x = 7.5/100 Cross multiply and solve for x. I think.

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