Molarity and Dilution
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Molarity and Dilution
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?
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Re: Molarity and Dilution
molarity of KCl(aq)=55.1g/(74.5513g/mol)/(0.125L)=5.91 mol/L (assuming 75 ml of water is an aliquot of water).
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Re: Molarity and Dilution
Why is this problem not a Mi * Vi= Mf* Vf, if it is giving you the final volume (125ml).
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Re: Molarity and Dilution
Molarity is moles of solute/volume of solution. The Mi*Vi = Mf*Vf is dilution, which is not what this particular problem is asking. To solve this problem, you first convert grams of KCl to moles by dividing the mass by the molar mass. Then, you divide the number of moles by the volume in liters, which would be 0.125L. The quotient would be your molarity.
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Re: Molarity and Dilution
So the 75 mL of the original solution is not required to solve the final molarity?
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Re: Molarity and Dilution
No, because it mentions how the final volume BECOMES 125 mL meaning it replaces the 75 mL. I think that part was meant to test us.
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Re: Molarity and Dilution
I would use the equation Minitial x Vinital = Mfinal x Vfinal, based on the information we are given.
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Re: Molarity and Dilution
If done correctly, should I have gotten 5.92?
I also was not entirely sure how to implement Mi x Vi = Mf x Vf
I also was not entirely sure how to implement Mi x Vi = Mf x Vf
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Re: Molarity and Dilution
I still don't understand why this isn't a dilution when a 75mL solution was created at first, then this same solution was diluted to get a 125mL solution. In what way would a dilution problem be written differently?
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Re: Molarity and Dilution
For this problem, you do not use M1V1=M2V2 because there’s no initial or final molarity or volume. It’s simply just moles/(total volume). The total volume being 125ml.
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Re: Molarity and Dilution
The molarity of the solution is the same in the 75 mL of water and the 125 mL of water. Since the molarity is the same, we are only looking for the molarity of the final solution. We can also find the molarity of the first solution by doing M=.74 Moles/.075L= 9.87 mol/L. The molarity of the first solution isn't necessary in this exercise, but it helps us conceptualize the fact that with the same amount of moles and less water, the concentration is higher than the second solution.
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