Rault's Law, Finding Mass of an unknown.

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Discussion Overview

The discussion revolves around a homework problem involving Rault's Law, specifically calculating the molar mass of an unknown solute based on vapor pressure changes when dissolved in water. The focus is on the mathematical approach and accuracy in calculations.

Discussion Character

  • Homework-related
  • Mathematical reasoning

Main Points Raised

  • The initial calculation attempts to determine the molar mass of the solute, X, based on the change in vapor pressure, with a reported answer of 227 g.
  • One participant suggests avoiding rounding down intermediate values to improve accuracy in calculations.
  • Another participant shares their experience of obtaining 227.7 g and expresses confusion about when to round values during calculations, noting variability in teaching practices.
  • A later reply emphasizes the importance of not rounding intermediate values, recommending the use of additional significant figures to maintain precision in calculations.

Areas of Agreement / Disagreement

Participants generally agree on the importance of maintaining accuracy in calculations by not rounding intermediate values. However, there is no consensus on the best practices for rounding in reporting results, as opinions vary among educators.

Contextual Notes

Participants discuss the implications of rounding on the accuracy of their results, highlighting the potential for discrepancies in final answers based on different rounding practices.

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Homework Statement



When 10.7 g of an unknown, non-volatile, non-electrolyte, X was dissolved in 100. g of water, the vapor pressure of the solvent decreased from 23.8 torr to 23.6 torr at 298 K. Calculate the molar mass of the solute, X.

Homework Equations



The answer is 227, but I know I'm doing wrong in the equation somewhere.

The Attempt at a Solution



100gH20 = 5.56 moles, Molar Fraction = .991

.991 = 5.56 / 5.56 + x =

5.51 + .991x = 5.56

.05 / .991 = 05 <---subtracted

10.7 / .05 = 214 g

Would appreciate any help!
 
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Don't round down intermediate values and you will be OK.
 
Borek said:
Don't round down intermediate values and you will be OK.

Sigh I just got 227.7, I would have definitely rounded off but at least I got the answer now lol. Generally speaking, should I ever round off during the calculations? Some teachers do and some don't so I'm a little confused.
 
Never round intermediate values during calculations (you have just seen why). If you have to report them, report rounded, but in calculations either use all digits you got (if they are kept in calculator register), or - if you have to save some value to use it later, use at least 2 or 3 so called guard digits - that is, if you know that the number you calculated has 3 sig figs, write it down with 5 or 6 digits.
 

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