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### Topic: solution problem  (Read 3619 times)

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#### Endeavor

• Guest
##### solution problem
« on: April 05, 2005, 01:59:37 PM »
I don't know how to start the process of answering this question:
"What is the molar mass of a nondissociating compound if 5.76 g of the compound in 750 g of benzene gives a freezing-point depression of 0.460 °C?"

I know Benzene has a molal freezing-point depression constant of 5.12 °C/m.

I don't know where to start....

#### charco

• Guest
##### Re:solution problem
« Reply #1 on: April 05, 2005, 06:26:34 PM »
deltaT(b.p.) =- k(b)m

where m is the molality fraction (moles/kg solvent)
change in boiling point = 0.46ºC
molal freezing-point depression constant of 5.12 °C/m = k

so 0.46/5.12 = molality fraction
so molality fraction = 0.0898 = moles unknown / kilograms benzene

is 750g benzene are used = 0.75kg
so 0.0898 x 0.75 = moles unknown = 0.0674

this has a mass of 5.76g

therefore molar mass = 85.45g

If you want a fuller explanation just Google "colligative properties freezing point depression"
« Last Edit: April 05, 2005, 06:30:09 PM by charco »

#### Endeavor

• Guest
##### Re:solution problem
« Reply #2 on: April 06, 2005, 12:34:33 AM »
thanks charco, i understand your explanation.