Question from Midterm Review

[David Perlman 1H]

Hello, this is question 2 part c of Sam's review workshop: "A solution is prepared by placing 8.124g of Na2CO3 into a 250.0 mL volumetric flask and filling the flask completely with water. c) 115.0 mL of this solution is dispensed into a 500.mL volumetric flask, which is then filled completely with water. What is the concentration of the new solution?"

Can someone please explain how they did this? I see in the answer key that the molarity from the 250.0 ml solution is the same molarity used for the 115.0 mL and I'm wondering why? Isn't the molarity different because the volumes are different?

[Alyssa Bercero 3H]

1. Convert mass of Na2CO3 into moles (g -> mol)
2. Use equation M=n/V (Molarity=moles/volume). The volume for this is the volume in which the Na2CO3 is first prepared in L (convert mL -> L, so 0.25 L).
3. Use equation M1V1=M2V2 and solve for M2 where M1 is the molarity from step 2, V1 is the volume that is dispensed into the 500 mL flask (115.0 mL), and V2 is the final volume of 500 mL.

[David Perlman 1H]

Thanks for your reply! So, why does the M1 still stay the same from the original solution considering that the volume (from 250 ml to 115 ml) is different?

[Alyssa Bercero 3H]

Thanks for your reply! So, why does the M1 still stay the same from the original solution considering that the volume (from 250 ml to 115 ml) is different?

When diluting a solution, the numbers of moles stays the same, so therefore the same initial molarity is used for the 250 mL and 115.0 mL flasks.

[David Perlman 1H]

Thanks for your reply! So, why does the M1 still stay the same from the original solution considering that the volume (from 250 ml to 115 ml) is different?

When diluting a solution, the numbers of moles stays the same, so therefore the same initial molarity is used for the 250 mL and 115.0 mL flasks.

But wouldn't the volume change, thus changing the molarity? Since molarity = moles/liter

[Alyssa Bercero 3H]

Thanks for your reply! So, why does the M1 still stay the same from the original solution considering that the volume (from 250 ml to 115 ml) is different?

When diluting a solution, the numbers of moles stays the same, so therefore the same initial molarity is used for the 250 mL and 115.0 mL flasks.

But wouldn't the volume change, thus changing the molarity? Since molarity = moles/liter

I think the molarity of a solution stays the same no matter how much solution is taken out and placed in a new flask because the molarity is distributed evenly throughout the flask. So if you have one beaker of solution, the molarity in that beaker is the same as the molarity in one spoonful of that beaker. Hopefully this makes sense.

[David Perlman 1H]

When diluting a solution, the numbers of moles stays the same, so therefore the same initial molarity is used for the 250 mL and 115.0 mL flasks.

But wouldn't the volume change, thus changing the molarity? Since molarity = moles/liter

I think the molarity of a solution stays the same no matter how much solution is taken out and placed in a new flask because the molarity is distributed evenly throughout the flask. So if you have one beaker of solution, the molarity in that beaker is the same as the molarity in one spoonful of that beaker. Hopefully this makes sense.

Okay, I understand the concept a lot better now! Thank you so much for your help