Respuesta :
Answer : The molar mass of naphthalene is, 128 g/mol
Explanation : Given,
Molal-freezing-point-depression constant [tex](K_f)[/tex] for benzene = [tex]5.12^oC/m[/tex]
Mass of naphthalene (solute) = 5.00 g
Mass of benzene (solvent) = 100 g = 0.100 kg
Formula used :
[tex]\Delta T_f=i\times K_f\times m\\\\T^o-T_s=i\times K_f\times\frac{\text{Mass of naphthalene}}{\text{Molar mass of naphthalene}\times \text{Mass of benzene in Kg}}[/tex]
where,
[tex]\Delta T_f[/tex] = change in freezing point = [tex]2.0^oC[/tex]
[tex]\Delta T_s[/tex] = freezing point of solution
[tex]\Delta T^o[/tex] = freezing point of benzene
i = Van't Hoff factor = 1 (for naphthalene electrolyte)
[tex]K_f[/tex] = freezing point constant for benzene = [tex]5.12^oC/m[/tex]
m = molality
Now put all the given values in this formula, we get
[tex]2.0^oC=1\times (5.12^oC/m)\times \frac{5.00g}{\text{Molar mass of naphthalene}\times 0.100kg}[/tex]
[tex]\text{Molar mass of naphthalene}=128g/mol[/tex]
Therefore, the molar mass of naphthalene is, 128 g/mol
