The number of units, which are used to express concentration of solutions from the following is _________
Mass percent, Mole, Mole fraction, Molarity, ppm, Molality

Solution

Chemistry ma’am · 3 steps

Ask of each term: does it compare solute with solution or solvent? Mole alone compares nothing.

1. What concentration means. A concentration unit tells you how much solute is present relative to the solution or the solvent. So it must always be a ratio of two amounts.

   Term	What it compares	Concentration unit?
   Mass percent	mass of solute per 100 g of solution	yes
   Mole	nothing: it is just an amount of substance	no
   Mole fraction	moles of a component per total moles	yes
   Molarity	moles of solute per litre of solution	yes
   ppm	parts of solute per 10⁶ parts of solution	yes
   Molality	moles of solute per kg of solvent	yes

2. Why mole fails. One mole of solute tells you nothing about how much solvent it is dissolved in. It could be in 1 L or in 100 L, so it cannot express concentration.
3. So. Mass percent, mole fraction, molarity, ppm and molality all work, giving 5.

The common slip: 6. Counting mole as a concentration unit gives 6. Mole is only an amount, so it is not a concentration.

The idea to keep. A concentration unit is always a ratio: solute against solution or solvent. A plain amount like mass or moles, with nothing to compare it to, is never a concentration.

Answer from the NTA final answer key. Solution by PrepWiser.