If a mass of 3.9g of the salt dissolved in 100g of water produces a solution that freezes at -1.40 degrees celsius , then what is the formula of this salt?
You don't have enough information. If we know the van't Hoff factor (i in delta t = i*Kf*m) we can calculate the molar mass of the salt but there is no way with this information that we can name the salt.
