Another Study Question by Chris

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Dear All,

wrote the following question another site and she also placed

a method that she used to do this problem.

" " " " Is there any other way to do the following problem?

Question #117 on practice test:

Which of the following formulas will produce a 1,000 ml solution of

3.5% amino acid in 15 % dextrose?

" " " " "

To which I responded the following and I transfer my post here.

Dear and ALL:

Well I can say that this is patterned after the formula that I have

in my Tutorials for an Allegation and C1 x V 1 = C2 x V2: it is a

reverse allegation and the reverse 'C1V' equation .

If you set up the allegation with zero or 0 in the lower left corner

to represent any solution that does not have the drug of the 3.5%

then you can work backwards.

another way is to use the old:

Desired strength divided by the Stock strength times the Desired

voume = Stock Volume to use

DS

--- x DV = SV

SS

3.5%

--- x 1000ml = 500ml

7 %

Then the formula would be 3.5% x 1000ml = 7% x 500ml

In which case one would use off the shelf 1000ml of a 3.5 % solution

or make it by mixing 500ml of a 7% solution with 500ml of WATER.

How did I get the amount of WATER? by Subtraction.

1000ml must be made. Using only 500ml of 7% is one half the volume

but twice the strength. Therefore the strength must be cut in half

and volume doubled.

Deisred Volume minus the stock volume = Volume of WATER for dilution

DV - SV = Water Volume

1000ml (of 3.5%) - 500ml (7%) = 500 ml WATER

So in YOUR language this is a 'reverse' dilution problem. It is

asked in another way.

Here is another problem which may be similar but not with it the idea

of one half or 1/2, and still using the same language in your

question:

'Which of the following formulas will produce a 1,000 ml solution of

3.5% amino acid in 15 % dextrose?'

a. 437.5 ml of 8% Amino Acid + 562.5 ml of 26.7% Dextrose

b.350ml of 10% Amino Acid + 650 ml of 23.08%

Dextrose

c. 785.72 ml of 4.45 Amino Acid + 214.29% of 70% Dextrose

The answer is all of them. Mathematically these formula all equal

1000 ml of 3.5% AA + D15W.

hint set the volume 1 times concentration 1 divided by the Desired

Concentration 1 equal to the desired strength 1. This will give you

the volume of 1000 ml V1 x C1/DC1 = DS1

Now set the volume 2 times concentration 2 divided by the

concetration equal to the desired strength 2 this also gives you

1000ml V2 x C2/ DC2 = DS2

Hint: You must also make sure that the Concentrations are greater

than the one to make. You can not dilute a weaker solution to make

the desire solution, you can only dilute a stronger to make a weaker.

Now if the Volume 1 + Volume 2 = 1000ml then the

answer is correct!

checking example a:

437.5 x 8% / 3.5% = 1000 ml

562.5 x 26.7% / 15% = 1000ml

_________________________________

1000 ml of 3.5%AA and 15% Dextrose

I hope that this helps you and others. I do not think that there will

be too many of these. But one never knows!

The real life purpose of this problem is to check the final formula

of a TPN Base Solution to see if the dilution is correct and meets

the doctors initial order.

Respectfully,

Jeanetta