Question

Chloroform boils at 61.3oC. What mass of naphthalene (C10H8 mw=128 amu) would be needed to raise the boiling point of 25.0 g of chloroform to 68.3oC if the boiling point elevation constant for chloroform is 3.63 oC/m B4 A) 6.2 g napthelene B) 19.3 g napthelene C) 0.635 g napthelene D) 2.09 g napthelene E) None of the above

Answer 1

boiling point of pure Chloroform:61.3 °C

The following formula can be used

Δt = i K_b m
Kb for Chloroform is 3.63 °C/m

Δt = 68.3°C - 61.3 °C= 7°C

i = 1 for non-electrolytes

m = molality = Z mol / 0.025 Kg

7°C = 1 x  3.63 °C/m x Z mol / 0.025 Kg

Z =0.0482 Mol

Mass of naphthalene to be added =  0.0482 Mol x 128 g/mol = 6.2 g

Hence 6.2 gm of naphthalene must be added raise the boiling point of 25.0 g of chloroform to 68.3°C

Option A is correct answer