Molality of the solution is defined as Number of moles of solute present in 1 Kg of solvent.
Mathematical it can be represented as:
m=n/ mass of solvent(Kg)
m=Molality of the solution,
n=Number of moles of solute
Here as it is given 10 g of NH4Cl and Molar mass of NH4Cl=53.491 g/mol
Number of moles of NH4Cl:
=Weight of solute/Molar mass of solute
=10/53.491
=0.186
Mass of solvent=Mass of water =12.7 g
So molality:
=number of moles of NH4Cl/ mass of water(Kg)
=0.186*1000/12.7
=14.7m
