Question

10 g of NaOH is dissolved in 250 ml water. Calculate the molality of this solution if density of pure water under experimental condition is 1g/ml.

Answer 1

Explanation :

We have given mass of NaOH = 10g

Volume of water = 250 ml

Density of water = 1 g/ml

What is molality of solution?

Mass of solution is given by,

★ Mass = Volume × Density

⇒ Mass = 250ml × 1g/ml

⇒ Mass = 250 g

⇒ Mass = 250/1000 kg

⇒ Mass = 0.25 kg

Now we know,

★ No.of moles = Given mass/Molar mass

★ Molar mass of NaOH = 40 g

Therefore,

⇒ No. of moles = 10/40

⇒ No. of moles = 0.25

Now we know that,

★ Molality = No.of moles / Mass of solution (kg)

⇒ Molality = 0.25/0.25

⇒ Molality = 1 mol/kg

∴ Molality of solution = 1 molkg-¹