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Posted: Sun Sep 29, 2019 11:22 pm
I understand that in a typical dilution problem (M1V1=M2V2) that the moles of the solute do not change. However, don't the moles of solute change when a portion of the sample is removed? Say if there is a 0.211M solution of 5g KCl and the rest water. And then 10mL is removed, wouldn't that 10 mL of solution removed have a different number of moles of KCL?
Posted: Sun Sep 29, 2019 11:29 pm
To answer your question, I had the same idea at first and was a little confused but the molarity does not change. Molarity is in moles per liter and is therefore constant across a whole solution once all of the moles of the solute and solvent have been combined. Therefore, when we remove a certain amount, this amount still has the same molarity because we the moles/liter is still constant. Similarly, the solution around will not change because the moles/liter will be the proportional in the amount left to before any was removed.
Posted: Sun Sep 29, 2019 11:39 pm
I understand why the moles and molarity stay the same now! But, follow up question; you would still change the volume in the M1V1=M2V2 equation? Because there is a new volume in the portion taken out right? Only the moles and molarity stay the same.
Posted: Mon Sep 30, 2019 1:41 am
Yes this is correct! M1V1 still equals the amount of moles and the amount of moles of a certain substance is not the same although the morality may be between two different solutions. So therefore we would use the new volume in the equation.