Question

4 g of caustic soda (molar mass 40) is dissolved in water and solution is made to 200 gm.calculate the molality of the solution​

Answer 1

molality=no of moles of solute/volume of solvent in L

no of moles=4/40=0.1

molality=0.1*1000/200

            =0.5mol/kg

Answer 2

Molality of the given solution is 0.5 m.  

Explanation:

It is known that number of moles of a substance are equal to mass divided by molar mass of the substance. Since, it is given that mass of caustic soda is 4 g and its molar mass is 40. Hence, its number of moles are as follows.

      No. of moles = $\frac{\text{mass}}{\text{molar mass}}$

                            = $\frac{4 g}{40 g/mol}$

                            = 0.1 mol

Also, molality is defined as number of moles present in kg of solution. As 1 gm equals 0.001 kg. So, 200 gm will be equal to 0.2 kg. Therefore, calculate the molality of given solution as follows.

           Molality = $\frac{\text{no. of moles}}{\text{mass in kg}}$

                         = $\frac{0.1 mol}{0.2 kg}$

                         = 0.5 m

Therefore, we can conclude that molality of the given solution is 0.5 m.

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