Finding Molecular Weight from Freezing Point Depression

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SUMMARY

The discussion focuses on calculating the molecular weight of a compound containing boron, nitrogen, and hydrogen based on freezing point depression. The user correctly rearranged the freezing point depression equation to find molality, then calculated the number of moles of solute by multiplying molality by the mass of the solvent (benzene). The final step involved dividing the mass of the compound (6.32g) by the calculated number of moles to determine the molecular weight. It is emphasized that the user calculated molality, not molarity, and that the molecular formula is not required for this calculation.

PREREQUISITES
  • Understanding of freezing point depression and its equation
  • Knowledge of molality and its calculation
  • Familiarity with the concept of molecular weight
  • Basic skills in stoichiometry and mass-to-mole conversions
NEXT STEPS
  • Study the freezing point depression equation in detail
  • Learn how to calculate molality and its applications in solution chemistry
  • Explore the relationship between molecular weight and empirical formulas
  • Investigate the use of cryoscopic constants in various solvents
USEFUL FOR

Chemistry students, researchers in physical chemistry, and professionals involved in solution chemistry and molecular weight calculations will benefit from this discussion.

Nicolaus
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A compound contains boron (40.3%), nitrogen (52.2%), and hydrogen (7.5%) - by mass. If 6.32g of this compound if dissolved in 100g of benzene, the solution freezes at 1.30 celsius. If the cryoscopic constant is 5.12 and freezing pt of benzene is 5.48, what is the molecular weight of compound?

I first rearranged the freezing pt depression equation to solve for the molarity of the solution (in mol of solute per kg of solvent), then multiplied this by the mass of of the solvent to obtain number of moles of solute. Being given the mass (6.32g) of the compound, I divided this mass by the number of moles calculated to obtain the molecular weight of the compound. Is this the correct approach?
I can calculate the actual molecular formula by using the % by mass and the molecular weight computed.
 
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Your description of what you did sounds OK - just remember you have calculated molality, not molarity.

Unless you have quoted only part of the problem, you are not asked to find the molecular formula, so you can safely ignore the composition information.
 

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