A solution contains 3.75 g of a nonvolatile pure hydrocarbon in 95 g acetone. The boiling point of pure acetone is 55.95°C and of the solution is 56.50°C. The Kb for acetone is 1.71°C/m. What is the molar mass of the hydrocarbon?

Respuesta :

Answer: The molar mass of the hydrocarbon is 37 g/mol

Explanation:

Elevation in boiling point is given by:

[tex]\Delta T_b=i\times K_b\times m[/tex]

[tex]\Delta T_b=T_b-T_b^0=(56.50-55.95)^0C=0.55^0C[/tex] = Elevation in boiling point

i= vant hoff factor =1  (for non electrolyte )

[tex]K_b[/tex] = boiling point constant = [tex]0.512^0C/m[/tex]

m= molality

[tex]\Delta T_b=i\times K_b\times \frac{\text{mass of solute}}{\text{molar mass of solute}\times \text{weight of solvent in kg}}[/tex]

Weight of solvent (acetone) = 95 g = 0.095 kg

Molar mass of solute = M g/mol

Mass of solute = 3.75 g

[tex]0.55^0C=1\times 0.512\times \frac{3.75g}{Mg/mol\times 0.095kg}[/tex]

[tex]M=37g/mol[/tex]

Thus the molar mass of the hydrocarbon is 37 g/mol

The molar mass of hydrocarbon will be "37 g/mol".

Given:

  • Boiling point of pure acetone, [tex]T_b^0 = 55.95^{\circ} C[/tex]
  • Boiling point of solution, [tex]T_b = 56.50^{\circ} C[/tex]
  • Elevation in boiling point, [tex]\Delta T_b = T_b - T_b^0[/tex]

                                                           [tex]= 56.50-55.95[/tex]

                                                           [tex]= 0.55^{\circ} C[/tex]  

  • Boiling point constant, [tex]K_b = 0.512^{\circ} C/m[/tex]
  • Van hoff factor, [tex]i =1[/tex]
  • Weight of solvent = 95 h or, 0.095 kg
  • Mass of solute = 3.75 h

As we know,

→ [tex]\Delta T_b = i\times K_b\times m[/tex]

or,

→ [tex]\Delta T_b = i\times K_b\times \frac{Mass \ of \ solute}{Molar \ mass \ of \ solute\times Weight \ of \ solvent}[/tex]

By putting the values,

→ [tex]0.55 = 1\times 0.512\times \frac{3.75}{M\times 0.095}[/tex]

     [tex]M = 35 \ g/mol[/tex]

Thus the above answer is correct.  

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