calculating molarity: The units tell you what to do

 Dear Dr Erskine,

  I am having some difficulty figuring out number 76, but have understood everything else fairly easily up to that point.  Would you mind giving me any pointers on that one?

V

Hey V,

Here's the problem,

Calculate the concentrations of each ion in a solution that results from mixing 50 mL of a 0.2 M NaClO3 solution with 25 mL of a 0.20 M Na2SO4 solution.

The units always tell you what to do. You want molarities, which is moles divided by liters, so get moles. Get Liters. Divide. Viola!

At least no reaction occurs in this problem, which would complicate things. And both compounds are soluble, so they break up completely into ions. More specifically:

 

First calculate the number of moles of ions in each of the two solutions prior to adding them together. (Liters times molarity). Realize the Na2So4 solution will give you twice as many moles of sodium ion as moles of sulphate (multiply by 2) to get moles Na for that one.)

 

Then add the moles sodium of both solutions together to get total moles sodium. the other moles should stay the same (sulfate and chlorate ions)

 

Add the two volumes together to get the new volume, convert to liters. Divide the new moles by liters to get molarities of each ion.

 

Let me know if that does not work.

Holly