What is the molar mass of a non-electrolyte compound if 4.28 grams is dissolved in 25.0 grams of chloroform solvent to form a solution which has a boiling point elevation of 2.30 degrees celsius?

Question

The boiling constant of chloroform is 3.63 degrees Celsius/m
im not sure if this is needed but molar mass of chloroform = 119.38 g/mol

I need a step by step explanation please.

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Answer 1 · 2017-03-27T00:39:15

The molar mass is 270 g/mol.

The formula for boiling point elevation #ΔT_"b"# is

#color(blue)(bar(ul(|color(white)(a/a)ΔT_"b" = K_"b"bcolor(white)(a/a)|)))" "#

where

#K_"b"# = the molal boiling point elevation constant of the solvent
#bcolor(white)(m)# = the molality of the solution

We can rearrange the formula to get

#b = (ΔT_"b")/K_"b"#

In your problem,

#ΔT_"b" = "2.30 °C"#
#K_"b" = "3.63 °C·kg·mol"^"-1"#

∴ #b = (2.30 color(red)(cancel(color(black)("°C"))))/(3.63 color(red)(cancel(color(black)("°C")))·"kg·mol"^"-1") = "0.6336 mol·kg"^"-1"#

You have 4.28 g of compound in 0.0250 kg solvent.

;: #b = "0.6336 mol"/(1 color(red)(cancel(color(black)("kg")))) = "4.28 g"/("0.0250" color(red)(cancel(color(black)("kg"))))#

Divide both sides of the equation by #0.6336#.

∴ #"1 mol" = "4.28 g"/0.0250 × 1/0.6336 = "270 g"#

The molar mass is 270 g/mol.