Question

Calculate the mass of a non-volatile solute (molar mass 40 g mol⁻¹) which should be dissolved in 114 g octane to reduce its vapour pressure to 80%.

Answer 1

What is Molar Mass?  

In chemistry, the molar mass M  a physical property which is defined as the mass of a given substance, element or l compound, divided by the amount or mass of the substance.

The basic  SI unit for molar mass is kg/mol.

However, historically, molar masses are expressed in g/mol.

In the problem, Ps=80% of P0=0.80.

Solute =W40gmol−1.

Solvent (octane)=114g114gmol−1,(molar mass of C8H18=114gmol−1).

Now P0−PsP0=x2∴P0−0.80P0P0=W40W40+1.

                    Or

0.8W40=0.2⇒W=10g is the mass of non-volatile solute