3. To prepare a very dilute solution, it is advisable to perform successive dilu

Question

3. To prepare a very dilute solution, it is advisable to perform successive dilutions of a single prepared reagent solution, rather than to weigh out a very small mass or to measure a very small volume of stock chemical. A solution was prepared by transferring 0.661g of K2Cr2OT to a 250.0-mL volumetric flask and adding water to the mark. A 1.000mL sample of this solution was transferred to a 500.0-mL volumetric flask and diluted to the mark with water. Then 10.0mL of this solution was transferred to a 250.0-mL flask and diluted to the mark with water. (a t is the final concentration of K2Cr207 in solution? (b) What is the mass of K2Cr20r in this final solution? (c) Could this mass be weighed out accurately using the balances in our lab?

Explanation / Answer

Let the molarity of solution 1 be M1

then M1 = (mass /molar mass) x(1000/VmL)

= (0.061g/294.18g/mL) x(1000/250)

=8.29x10-4 M

1 mL of this solution is diluted to 500mL whose molarity is M2

thus M1xV1 = M2xV2

1mL x 8.29x10-4 M = M2 x 500mL

Thus M2 = 1.65x10-6 M

Now 10 mL of this solution is diluted to 250mL

molarity of this final solution = 10mLx1.65x10-6 M/250mL

= 6.6x10-8 M

b) mass of dichromate in the solution = molarity x molar mass /V(L)

=6.6x10-8 M x294.18g/mol /0.25L

=4.89x10-6 g

c) It is not possible to weigh such small amount of mass in the laboratory.

That is the reason we chose multiple dilutions to prepare thatdilute solution.

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