Question

First, you prepare a more concentrated solution of the reagent. Then, you prepare a more dilute solution by performing one or more dilutions. For example, suppose that you prepare a stock solution by placing 1.123 g K2Cr2O7 in a 1000-mL volumetric flask and dilute to volume. Next, 10.00 mL of this solution is placed in a 250.0-mL volumetric flask and diluted to volume. Finally, 1.00 mL of this solution is placed in a 100.0-mL volumetric flask and diluted to volume. What is the molar concentration of the last solution?

Answer 1

This is easy.
Molecular weight of K2Cr2O7 = 294.185g/mol
Find the moles using the formula
mol = grams/molecular weight of compound

thus

mol = 1.123g/ 294.185g/mol
=
0.0038173 mol

once you have the moles use the formula
Molarity = moles/Liters

0.0038173 mol/1
= 0.0038173M

Now look for your dilution just use ratios and multiply by your molarity if it gets larger you are doing it wrong, if the concentration gets smaller that you are doing it right.

0.0038173M(10mL/250mL)(1.00 mL/100mL)
= Concentration of your last soultuon
= 0.00000152692M or 1.52692 × 10^-6M (I would use scientific notation for this solution, most profs will dock marks if you don't)

Enjoy