Molarity given percent solution and density.

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Discussion Overview

The discussion revolves around calculating the molarity, molality, and mole fraction of a lactose solution given its percent concentration and density. The focus is on the mathematical approach and unit conversions necessary for these calculations.

Discussion Character

  • Homework-related
  • Mathematical reasoning

Main Points Raised

  • One participant presents a problem involving a 12.0% lactose solution with a density of 1.06 g/mL and seeks to determine its molarity, molality, and mole fraction.
  • Another participant suggests that the first step is to calculate how many grams of lactose are in 1.06 grams of solution.
  • A later reply emphasizes the importance of showing units at each step to ensure correct conversions.
  • One participant attempts to calculate the moles of lactose using the formula but questions whether their approach is correct.
  • Another participant acknowledges the calculation but reminds to keep track of the units throughout the process.
  • A subsequent post questions the context of the moles of lactose being calculated, indicating a need for clarity in the calculations.

Areas of Agreement / Disagreement

Participants generally agree on the need for careful unit conversion and tracking throughout the calculations. However, there is no consensus on the correctness of the calculations presented, and some aspects remain unresolved.

Contextual Notes

Limitations include potential misunderstandings in unit conversions and the need for clarity regarding the context of the calculations, particularly in relation to the total volume of the solution.

webz
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Homework Statement


Lactose (C12H22O11; MM=342) is a sugar found in mamalian milk. The density of a 12.0% solution is approximately 1.06 g/mL. What is the molarity, molality, and mole fraction of the solution?


Homework Equations


M=g/L, m=mol/kg, molefraction=mol/mol??


The Attempt at a Solution


I think I can get this one if I can just determine the molarity. I definitely did something wrong.

M=mol/L
1.06 / 1000 = .00106; .00106/342 = 3.099x10^-6M
I figured since I know g/mL as well as mol/g, I can divide to get mol/mL, then convert to get L. Apparently this is not the case?
 
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You're on the right track, just a few things you need to pay attention to:

1.06 grams of solution contains how many grams of lactose?

Also, show your units at all steps of the calculation to make sure you're converting correctly.
 
Last edited:
Hmm, then let me try this again. I just want to find the moles of Lactose for now, then I think I got the rest.

1.06 g/mL * 1mol/342g * 12% lactose = 3.719x10^-4

Does that look right?

Thanks for the help btw.
 
That looks good, but remember to keep track of the units.
 
webz said:
I just want to find the moles of Lactose for now

Moles of lactose in what?

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