How to get molality without using a molecular formula?

In summary, a student conducted an experiment to determine the molar mass of acetone by using the method of freezing point depression. The equilibrium temperature of water and ice was found to be 1°C and after adding 11.1g of the sample, the temperature dropped to -3°C. The mass of the solution was 90.4g. The question is asking for the molality of acetone using the experimental data and the equation ΔTf = mkf. The student is unsure how to find the moles without using the molar mass of acetone. They know that ΔTf is 4°C and that m can be calculated by dividing moles of solute by kg of solvent. The mass percentage of
  • #1
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Homework Statement


Freezing point depression question[/B]
student determines molar mass of acetone using the method used in this experiment. She found that the eq temp of water and ice was 1 deg C. She then adds 11.1g of the sample to mixture and temp falls to -3 deg C. Mass of solution was 90.4g
What is the molality of the acetone using the experimental data and equation delta T_f = mk_f?

Homework Equations


Im lost on how to get moles without using the MM from formula for acetone?

The Attempt at a Solution


I know that T_f = 4deg C
also that m = moles solute / kg solvent
Also (if this is useful) that mass % = 12.28% acetone
also that kg of the solution will be = .904kg
thanks for any help
 
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  • #2
You know ΔTf, you know (or can look up) kf, so you can work out m. If you also know the MW of acetone, you can check how accurate this m value is.
 

FAQ: How to get molality without using a molecular formula?

1. How do you calculate molality without using a molecular formula?

Molality can be calculated by dividing the moles of solute by the mass of solvent in kilograms. This can be determined by using the molar mass of the solute and the density of the solvent.

2. Can molality be used instead of molarity?

Yes, molality can be used as an alternative to molarity for expressing the concentration of a solution. However, molality is more commonly used when dealing with solutions at high temperatures or when precise concentration measurements are needed.

3. What is the difference between molality and molarity?

Molality (m) is a measure of concentration that is based on the number of moles of solute per kilogram of solvent, while molarity (M) is based on the number of moles of solute per liter of solution. Molality takes into account the mass of the solvent, while molarity does not.

4. How do you convert from molality to molarity?

To convert from molality to molarity, you need to know the density of the solution. Then, you can use the formula M = m / (1 - (m * ρ)), where M is the molarity, m is the molality, and ρ is the density of the solution in grams per milliliter.

5. Is molality affected by changes in temperature or pressure?

No, molality is not affected by changes in temperature or pressure. This is because molality is based on the mass of the solvent, which does not change with changes in temperature or pressure. Molarity, on the other hand, is affected by changes in volume due to changes in temperature or pressure.

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