The mole fraction of a solvent in aqueous solution of a solute is...

Since mole fraction of solvent is $0.8,$ it implies that $10moles$ of aqueous solution will have $8$ moles of water and $2$ moles of solute.

$∴ 2moles$ of solute is present in $\to 8moles$ of water

Or, $2moles$ of solute is present in $\to$\left(8 \times 18\right)gm$of water</p><p>$144gm$of water has$2$moles of solute<br />$1gm$of water will have$\dfrac{2}{144}moles$of solute<br />$1kg$of water will have$\dfrac{2}{144} \times 1000 = 13.88moles$</p><p>$∴$Molality =$13.88m