Answer: 186 g/mol
Explanation:
Weight of solvent (water)=100 g = 0.1 kg (1 kg=1000 g)
Molar mass of solute (impurity) = ?
Mass of solute (impurity) added = mass of solution - mass of solvent (water) = (118- 100) = 18 g

= change in freezing point
= freezing point constant for water = 



The molecular mass of the impurity is 186 g/mol.
The best way is by heating the solution. The liquid will evaporate leaving the solid, such as salt, behind.
Answer : The number of moles present in ammonia is, 70.459 moles.
Solution : Given,
Mass of ammonia = 
Molar mass of ammonia = 17.031 g/mole
Formula used :


Therefore, the number of moles present in ammonia is, 70.459 moles.