I could do these not too long ago but my brain has recently lost the ability. I'm trying to study for my National Certification or whatever.

If a physician orders 25% dextrose 1000 ml and all you have is 70% dextrose 1000 ml, how much 70% dextrose and how much sterile water will be used?

If 200 grams of dextrose is ordered using a 50% dextrose solution, how many ml are needed?

How much talc is needed for an order for 120 gm of the following compound:
nupercainal ointment 4%, zinc oxide 20%, talc 2%?

I know the exam is multiple choice but these questions come in the practice exams I have. I need to know how to do them and be able to understand them for that purpose. The books I have to reference aren't too clear and do not explain these sorts of things too well.

Well for the first one, a 25% dextrose solution of 1000 ml is really 250 ml pure dextrose per 1000 ml. A 70% solution means you need more than 250 ml of it. Since it's multiple choice, I'd just use that to rule out a couple answers, and then plug in the logical ones to see which one fits.

Not sure about converting grams and ml in the 2nd one. For the 3rd,120 gm x .02 = 2.4 gm talc. I think that'd be right.

500ml of 70% solution + 500ml water = 1000ml 35% solution. so then 50ml = 3.5%. 3.5% into 25% = 7.1. 7.1 x 50ml = 357.

so 357ml of 70% dextrose and 643ml water.

i failed maths?

Ok, I think the bolded part is right so far. But then 50 ml is one fifth of the 1000 ml, so dividing 35% by 5 = 7% ...hovever now I am thoroughly confused :P I'm not the best at maths either.

ok... I'm a physiology student and my boyfriend is a pharmacology student and neither of us can remember how to do this... lol, not good with exams being only 3 weeks away...

If a physician orders 25% dextrose 1000 ml and all you have is 70% dextrose 1000 ml, how much 70% dextrose and how much sterile water will be used?

wouldnt it be something along the lines of (25/100)X 70 = 17.5 (17.5 X 1000)=17500 so it would be 1000ml of dextrose an 7400 of water. but that seems well out.

ok... I think I've found the right set of lecture notes... I've found these 2 examples so far of similar types of calculation... my maths fails, but I thought I'd put them up in case they help anyone work anythi gout...

I'll keep looking for the exact manor for doing this

Quote:

General formula for solving: Amt1 x %1 = Amt2 x %2
Amt1 = the quantity or amount of the ORIGINAL preparation.
%1 = the % strength of the ORIGINAL preparation expressed as a decimal or percent
Amt2 = the quantity or amount of the WANTED preparation
%2 = the % strength of the WANTED preparation expressed as a decimal or percent
To solve concentration and dilution problems you need to identify the two preparations in the equation, convert ratio or percentage strengths to decimal expressions and convert to same systems of measurement.

EXAMPLE: If 500 ml of a 15% solution are diluted to 1500 ml, what will be the percent strength?

Amt1 x %1 = Amt2 x %2
Step 1: Identify the two preparations in the problem and assign values to appropriate terms.
Step 2: Solve the equation by multiplying and solving for X.
500 ml x 15% = 1500 ml x X%
7500 = 1500X
7500/1500 = 1500X/1500
X = 5%

Quote:

If 8 ml of phenol were added to 480 ml of lotion, what is the percentage of phenol in the lotion?
8 ml (Active Ingredient)

(Total Amt) 480 ml X %
Divide to solve for X.
8 / 480 = X

EXAMPLE: If 1.2 gm of menthol is added to 480 ml of lotion, what is the percentage of menthol in the lotion?

1.2 gm (Active Ingredient)
(Total Amt) 480 ml X %
Divide to solve for X.
1.2 / 480 = x
x = 0.0025