Question

The molar concentration (M) of a solution prepared by dissolving 0.2362g of Cr(NO3)3 in a 50-mL volumetric flask is 0.01985M, where the molecular weight for Cr(NO3)3 = 238.01g/mol.
a. Suppose you want to prepare another solution containing chromium nitrate that is 25 times LESS concentrated than the one prepared above. Given a choice of 10-mL and 5-mL pipets and 50-mL and 100-mL volumetric flasks, explain how you would proceed in preparing the new diluted solution. In addition, calculate the concentration for the new diluted solution. SHOW ALL WORK. Your final value should have the correct unit and number of significant figures. Hint: You will most likely need two dilution steps in order to obtain the desired concentration. Note: You may NOT reuse the same pipet or combine different pipets within the same dilution step. You may reuse the pipet and/or volumetric flask in the different dilution step.

Answer 1

Answer:

Use a 10 mL pipet and a 50 mL volumetric flask, twice

Explanation:

Given those conditions, in order to have a solution that is 25 times less concentrated that the one previously prepared, we would need two dillution steps, as suggested by the problem.

If we take an aliquot of 10 mL and dilute it to a final volume of 50 mL, we prepare a solution that is 5 times less concentrated. And if we then take again an aliquot of 10 mL of that dilluted solution and dilute it to a final volume of 50 mL, we would have a solution 25 (5 times 5) times less concentrated than the original one.

In order to verify, we can calculate the resulting concentration and compare it with the original one:

$0.01985M*\frac{10mL}{50mL} *\frac{10mL}{50mL}=0.000794M$

Thus we can see that the obtained concentration is in fact 25 times lower than the original concentration.