Question

If the density of 3M solution of NaCl is 1.25 g/ml. then calculate the molality of the solution. detailed answer.

Answer 1

⭕️For 3M solution NaCl means, 3 moles of NaCl in 1 L of solvent

➖️By using the density we can find out the mass of the 1 L of solution. 

⭕️mass of solution= volume x density = 1000 ml (1 L) x 1.25 g/ml

⇒ 1250 g.

⭕️mass of solute = mass of 3 moles of NaCl =  58.5 g/mol  x  3 mol.

⇒175.5 g.
(molal mass of NaCl = 58.5 g/mol)

⭕️∴ mass of solvent = mass of solution - mass of solute = 1250- 175.5 gm = 1074.5 gm
⇒ 1.07 Kg.

⭕️∴ molality = moles of solute / mass of solvent = 3 mol/ 1.07 kg  
⇒ 2.8 mol kg-¹

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Answer 2

• Molar Mass of NaCl = Na + Cl

=> 22.98 + 35.45

=> 58.43 u

Density = 3M

Mass of 1 lit. solution = 1.25 g/ml

=> 1.25 × 1000

=> 1250 g (Mass of solution)

Now..

There are 3 moles of NaCl in 1 lit. of solution.

=> 3 × 58.43 = 174.29 g (Mass of solute)

Now..

》 Mass of solution = Mass of solvent + Mass of solution

Mass of solvent = Mass of solution - Mass of solute

=> 1250 - 174.29

=> 1075.71 g

=> 1075.71/1000

=> 1.07571 kg

=> 1.07 kg (approx.)

We know that

Molality = Moles of solute/Mass of solvent

=> 3/1.07

=> 2.8 mol kg-¹

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