ok, lets try another way. since neostigmine 1:2000 is 1 gram in 2000 ml do a little math to change that into milligrams. the ratio 1:2000 can also be written as a fraction with 1 in the numerator and 2000 in the denominator. divide the denominator into the numerator to obtain a decimal. doing the division and keeping the labels in the numerator and denominator you'll get 0.0005 grams/1 ml. since the dose you need is in milligrams you need to convert 0.0005 grams to milligrams. simply multiply it by 1000 to get 0.5 mg. keep in mind that now that you've reduced this fraction it now represents 0.5 mg per 1 ml
. because in dividing the numerator into the denominator you, in effect, made the denominator the number "1". now, using the formula of dose desired
divided by the dose you have just determined that you have on hand
multiplied by the amount the dose on hand comes in
you get 0.5 mg
neostigmine (dose desired) divided by the dose on hand 0.5 mg
neostigmine (dose on hand) multiplied by 1 ml
(that's the amount i determined in the calculations above that the dose on hand comes in). both the 0.5 mg in the numerator and the 0.5 mg in the denominator factor out to give you the number 1 and all you are left with is 1 times 1 ml. the answer is 1 ml
. if you compared what was just done here with the way i worked the problem by dimensional analysis you will see that everything was done with the exception that with demensional analysis you set up an equation of multiplication where you deliberately manipulate the fractions (ratios) so that you factor out labels, include conversion factors so you can get from grams to mg and are ultimately left with an answer with the label of ml, an amount, which is what the problem is asking for.
dimensional analysis is not a u.s. thing. it is mostly used by scientists in chemistry and physics (there are a few of them in canada, i presume) to calculate amounts of chemicals and elements that are needed in reactions. it is a method that adapts very well to working medication problems. you can see an interactive video for chemistry students on how dimensional analysis (or, factor labeling) works at this website
- click on "1.4 dimensional analysis". it is an interactive program that explains da (dimentional analysis)
using animation rather than video in what i thought were very simple terms. also includes several problems you can try your hand at which are not chemistry related but utilize the concepts of da.