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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?
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.
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.
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.
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