Question

If ten drops of 0.3 M HCl are mixed with ten drops of 0.3 M Na2S2O3, what will be the concentration of each chemical in the mixture?
Please help, i am pretty sure that the concentration should be 0.3 M but i am not sure if i am supposed to do somthing with the ten drops.
Thank you for any help

Answer 1

For clarity, just assign some number to the volume of a drop. Actually, a drop is between 0.03 and 0.05 mL (depending upon the size of the opening AND on the material that the buret/pipet/dropper is made). BUT, let's just make things simple by saying a drop is 1 mL. We know that isn't true but it doesn't matter as long as we are consistent.
So 0.3 M x 10 mL(0.01 L) = 0.003 mols HCl.
and 0.3 M x 0.01 L = 0.003 mols Na2S2O3.
Molarity HCl = #mols HCl/L solution = 0.003/0.02 L = 0.15 M. (The volume os 10 mL + 10 mL = 20 mL = 0.02 L)

Molarity Na2S2O3 = 0.15 by the same reasoning.
Of course, we should have know, just basically, that 0.15 M is the answer.
We add 10 drops 0.3 to 10 drop 0.3 so each 0.3 has been diluted by a factor of 2 so the final concentration is 0.3/2 = 0.15. I hope this is clear.

Answer 2

thank you very much