how many mL does he need?

Question

doctor has ordered 325 mg of a medication . The medication is available as a solution that contains .50 g of the medication per mL. How many mL of the solution would the patient need ?

I have (325 mg)/(.50ml) = 650

Answer 1

I would set this up using the unit analysis method. Your final answer is in mL. If I start with the expression " = ? mL " and then work "backwards" to make the units cancel I will arrive at the correct answer. The givens are 0.5 g per 1 mL and 325 mg, therefore the first term to the left of the equal sign is  (1 mL/0.5 g), the second term is the other given (325 mg). If we multiply the first and second terms  we have units of (mg x mL)/g, therefore we need another term to "change" units. The term must have the units of g/mg. That relationship comes from knowing 1 g = 1000 mg or 1 g per 1000 mg.

 

Final equation is then:

 

(1 g/1000 mg)(325 mg)(1 mL/0.5 g) = 0.65 mL

 

Do the units cancel properly?

Is the answer physically reasonable? 

Yes on both accounts.

The unit analyis method is quite powerful and longer more complicated problems can be solved with this method.

Answer 2

Since the concentration of the medication is given in g/ml (0.5g/mL) you need to divide the 650 by 1000 to get the same units. Or start from 0.5g/mL = 0.5g/mL*1000mg/1g = 500mg/mL

You need a 325mg dose:

X = (325mg)(1mL)/500mg = 0.65mL

 

Answer 3

Are your units correct?  If so, you need to adjust for the fact that the solution is in GRAMS/ml, not mg/ml.

 

You would need:

 

(325mg)(1ml/.5g)(1g/1000mg) = 0.65 ml