Question

What mass of solid that has a molar mass 89.0 g/mol should be added to 100.0 g of benzene to raise the boiling point of benzene by 2.42°C? (The boiling point elevation constant of benzene is 2.53°C•kg/mol.) 2.84 g 8.52 g 21.5 g 93.0 g

Answer 1

On the basis of given data:

Mass of solvent (benzene) = 100 gm or 0.1 Kg

Molar mass of solute (solid) = 89 g/mol

Increase in boiling point (deltaTb) = 2.42 degree C

The boiling point elevation constant k = 2.53 C.kg/mol

There is a need to determine the mass of the solid to be added, the elevation in boiling point is proportional to the m (molality) of the solute;

ΔTb = k.m

Here, m that is molality = moles of solute/kg of solvent

Therefore,

ΔTb = k (benzene) × moles of solid/kg of benzene

2.42 = 2.53 × moles of solid/0.1

moles of solid = 0.0956 moles

The molar mass of solid = 89 g/mol

Thus, the mass of solid = 0.0956 moles × 89 g/mol

= 8.508 g

Thus, the mass of solid to be added is 8.508 g.