[Atome]

Hello:

I answered the following problem correctly after erring once. However, I do not understand why the quantity of solution (highlighted in red) for the molarity and molality fractions are different in this case. Could someone please explain?

In other words, I do not understand why I had to subtract the mass of ammonium chloride from water to calculate the molality; whereas for molarity, I simply used 100 g as the quantity of water.

Thank you very much.

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1. An aqueous solution is 8.50% ammonium chloride, NH₄Cl, by mass. The density of the solution is 1.024 g/mL. What are the molality and molarity of ammonium chloride in solution?

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Molality:

Since this is 8.50% by mass:

Mass of NH₄Cl = 8.50 g -> 0.159 mol

Mass of solution = 100 g

Mass of H₂O = 91.5 g

Therefore: Molality = 0.159 mol/0.0915 kg = 1.74 molal

Molarity:

Since the solution is 8.50% by mass:

Mass of NH₄Cl = 8.50 g -> 0.159 mol

Mass of solution = 100 g

Therefore:

Mass Fraction = 8.50 g/100 g

= 0.159 mol/100 g = 0.159 mol/97.66 mL

Molarity = 0.159/0.0977 L = 1.63 M

[Borek]

whereas for molarity, I simply used 100 g as the quantity of water.

Quantity of the SOLUTION, not water.

[Atome]

Thank you for you reply, Borek.

I still do not understand why the quantity of the solution differs for the molality and molarity values. One is just measuring the mol of solute/kg of solution while the other represents mol of solute/L of solution.

Shouldn't I start with 100 g of solution in the beginning?

[Borek]

Quantity of solution is identical, but molality doesn't use amount of solution - it uses amount of solvent.

You start with 100 g of solution in both cases, just molarity is concentration per amount of solution while molality is concentation per amount of solvent. They have different definitions, that's all.

Solution amount is sum of amounts of solvent and solute.

No idea which point you are missing, so I can't help further.

[DrCMS]

Molality is moles of solute per kg of solvent
Molarity is moles of solute per L of solution