Assume that you did the following dilutions
1) you take 1ml of 10^-6 m of ascl3 and diluted it to 1 kilometer (solution a)
2) you the take 1 micrometer of solution a and dilute 10l to make solution b
3)then you take one ml of solution b and dilute to 1l to make solution c
find the concentration of ascl3 in solution c

The concentration of AsCl3 in solution C is  

Explanation:

To solve this problem we need the next equation

(final concentration)(final volume) = (initial concentration)(initial volume) or

(Cf)(Vf)=(Ci)(Vi)

and we are going to work in liters, so be aware of the units

For all the steps we need to find the final concentration, Cf, so:

1)

   Note that 1mL = 0.001L and 1kL = 1000L

   [A] =

2) now Ci = [A] so:

   Note that 1uL =

   [B] =

3) now Ci = [B] so:

   [C] =

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nacl is added to the aqueous of what? ? in order to get a proper answer you have to specify the exact composition of the aqueous solution.