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.)

Answer 1

Mass of benzene (solvent) = 100.0g = 0.1 kg

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

Elevation in b.pt (ΔTb) = 2.42 C

B.pt elevation constant k = 2.53 C.kg/mol

To determine:

The mass of the solid to be added

Explanation:

The elevation in b,pt is proportional to the molality (m) of the solute ;

ΔTb = k.m

where m = molality = moles of solute/kg of solvent

In this case we have:

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, mass of solid = 0.0956 moles * 89 g/mol = 8.508 g

Ans: Mass of solid to be added is 8.508 g