Question

The molality of pure water is _____

(a) 55.5

(b) 50.5

(c) 18

(d) 60.5

Answer 1

Answer : a) 55.5 moles/kg

Explanation :

Step 1 :

For a pure water the density is 1 gm/cc. In other words the mass is equal to the volume.

Density (gm/cc) = mass (gm)/ Volume (cc)

i.e 1 (gm/cc) = 1(gm) / 1 (cc)

i.e 1 Kg = 1000 gm

Step 2 :

no.of moles of pure water 

    n = 1000/18

Molality (m) = Number of moles/kg of solvent

m = 55.55/ kg of solvent

m = 55.5 moles/kg

a) 55.5