Find volume of each solution from total volume containing given molarity of each solution

Question

Given stock solution of glucose (1000mM), asparagine (100mM) and NaH2PO4 (50mM), how much of each solution do you need to prepare 500mL of a reagent that contains 50mM glucose, 10mM of asparagine and 2mM of NaH2PO4.

Answer 1

Sharine,

You need to write some equations based on the given information.

 

Remember that the concentration * the volume of solution gives you the amount of pure substance, and the amount in has to equal the amount out. Pick variables:

 

g = L of glucose solution

a = L of asparagine solution

M = L of monosodium phosphate solution

 

      Desired pure substance = input pure substance

                (given)                        ((find)

 

glucose      (50mmol/L)(.5L) = (1000mmol/L)(g)

     (25mmol)/(1000mmol/L) = g

                                .025 L = g

                                25 mL = volume of glucose solution

 

asparagine (10mmol/L)(.5L) = (100mmol/L)(a)

        (5mmol)/(100mmol/L) = a

                                 .05 L = a

                               50 mL = volume of asparagine solution

 

monosod...  (2mmol/L)(.5L) = (50mmol/L)(M)

          (1mmol)/(50mmol/L) = M

                                 .02 L = M

                               20 mL = volume of monosodium phosphate solution

 

Notice that these three volumes do NOT add up to 500mL. That's ok! The point was only to get the right amount of pure substance into the reagent. I hope this helps.

Bryan