Molarity Problem (#16)

[Nandini_Parmar_1I]

I am not sure how to go about this problem. Does it require us to take the difference between the initial and final volume of the solvent? ie. 125-75 or are they separate.

16. A solution is prepared by dissolving 55.1 g of KCl in approximately 75 mL of water and then adding water to a final volume of 125 mL. What is the molarity of KCl(aq) in this solution?

A. 0.276 mol.L-1

B. 5.91 mol.L-1

C. 98.5 mol.L-1

D. 3.70 mol.L-1

[Chris Oh 2I]

It just means that 50 mL of water is added, leading to a final volume of 125 mL!

I believe the question is just asking what the molarity of the overall solution is when that 50 mL of water is added to the original solution (so basically finding the molarity at 125 mL only!).

[Kainath Kamil Dis 2K]

Hi!
The final volume of the solution is 125ml. To find the molarity of the final solution, convert the grams of KCl (55.1g) to mols then divide it by the final volume (250ml). The 75mL seems to be throwing some people off because I don't think it's essential info.

Kainath Kamil Dis 2K

[Desiree Eshraghi 3A]

We have to break this problem up. 55.1 g refers to the initial moles of the solute (once you covert it appropriately). The initial volume 75 ml is irrelevant as the problem merely ask for the final molarity, not the DIFFERENCE in molarity. Therefore, all you have to do is divide the moles (which is found from converting 55.1g) by the final volume 125ml. It has extraneous information - which is why you have to read these sort of problems closely!

[Rachel Fox - 3F]

To solve this problem, you can approach it using the equation M=mol/L. To use this approach, find the moles of KCl (55.1g/74.5g.mol^-1), and then divide that value by .125L to get the molarity. The 75mL is not necessary for this problem because if you used the 75mL and the equation MinitialVinitial = MfinalVfinal, you would get ((55.1g/74.5g.mol^-1)/.075L)(.075L) = Mfinal(.125L). The .075L cancel out, as the equation MinitialVinitial = MfinalVfinal is equivalent to the equation n initial = n final, which then makes that equation become a rearrangement of M=mol/L.

[Grace Chen 3F]

I think you only need to calculate the number of mols in 55.1g of KCl, and divide the number of mols by 125ml (number of mols / final volume). You don't need to calculate the difference between the initial and final volume, because it is not important in terms of this question (adding water did not change the number of KCl mols in the solution, it only affected the final volume (which was given: 125ml))
*you only need the number of mols of KCl and the final volume to solve this question*