Question

What will be the molality of the solution prpared using 500 gm of 25 % w/w NaOH and 500 gm of 15 % w/w NaOH solution ? (Molecular weight of NaOH = 40 gm/mole)

(a)  12.74 m

(b)  6.25 m

(c)  9 m 

(d)  5 m

Answer 1

Correct option  (b) 6.25 m

Explanation:

mass of NaOH in 500 gm 25% W/W NaOH solution

= 5 x 25 = 125 gm. and

mass of H₂O = 5 x 75 = 375 gm.

mass of NaOH in 500 gm 15% W/W NaOH = 5 x 15 = 75 gm.

and mass of H₂O = 5 x 85 = 425 gm.

mass of NaOH in a mixed solution when both solutions are mixed W = 125 + 75 = 200 gm.

and mass of H₂O w₀ = 375 + 425 = 800 gm.

Now molality of mixed solution = 1000 x w/M x W₀ = 1000 x 200/48 x 800 = 6.25 m