## Study Module Post-assessment Question

AhYeon_Kwon_2H
Posts: 48
Joined: Fri Sep 28, 2018 12:27 am

### Study Module Post-assessment Question

I don't know what I did wrong for one of the study module questions for "Molarity and Dilution of a Solution":

5.00 g of KMnO4 is dissolved in a 150.00 mL flask of water. If 20.00 mL of this solution is removed and placed in a new 2nd 250.00 mL flask and filled with water, what is the concentration of the solution in the 2nd flask?
K (39.10 g/mol)
Mn (54.94 g/mol)
O (16.00 g/mol)

I'm pretty sure the equation to use is MINITIALVINITIAL = MFINALVFINAL, but I'm not sure what volume to use. Would it be the 150.00 mL or 20.00 mL? Or am I completely off track?

Thank you! :)

Meachelle_Lum_1I
Posts: 92
Joined: Fri Sep 28, 2018 12:24 am

### Re: Study Module Post-assessment Question

Hi! You're definitely on the right track. :)
The initial volume would be 0.02L because that's the amount you're actually bringing in to the new flask.
Even though we technically started with 0.15L, that was only used to calculate the number of moles of our solute, which stays constant throughout the problem :) We don't take 0.15L into our new flask however, so it's not the initial volume.
Hope this helps!

Nicolette_Canlian_2L
Posts: 77
Joined: Fri Sep 28, 2018 12:25 am
Been upvoted: 1 time

### Re: Study Module Post-assessment Question

I am confused on this problem as well. I'm unsure of what the initial and final volumes are. I used the 15L as the initial volume and when it came to the final volume I was not sure whether to use the 0.02L or 0.250L.

AhYeon_Kwon_2H
Posts: 48
Joined: Fri Sep 28, 2018 12:27 am

### Re: Study Module Post-assessment Question

Oh, okay! Thank you! :)

Andrew Lam 3B
Posts: 30
Joined: Fri Sep 28, 2018 12:24 am

### Re: Study Module Post-assessment Question

MinitialVinitial = MfinalVfinak

Minitial = 0.2109 M
Vinitial = 0.02 L (Only 20.0 mL is transferred)
Vfinal = 0.250 L

Solve for Mfinal, which should be 1.69 x 10-2 M

Katelyn Phan 2A
Posts: 61
Joined: Fri Sep 28, 2018 12:23 am

### Re: Study Module Post-assessment Question

Nicolette_Canlian_3G wrote:I am confused on this problem as well. I'm unsure of what the initial and final volumes are. I used the 15L as the initial volume and when it came to the final volume I was not sure whether to use the 0.02L or 0.250L.

The final volume would be .250 ml since it's what you're transferring and attempting to dilute the concentration into. I used the key concept n(initial) = n(final) and it made the problem much easier to solve. You can ignore the original amount and simply find the number of mols in 5.00g of KMnO4 by dividing the mass by the molar mass. Then you can set n(initial) = M(final)V(final). Use .250ml as V(final) and solve for M(final)
Hope this helps!! :)