Question

Why does solution D have the highest ionic strength? A) 0.1M H3PO4 B) 01.M KH2PO4 C) 0.1M K2HPO4 D) 0.1M K3PO4

Answer 1

for D )

ionic strength = 1/2 [m1z1^2 + m2 z^2 ]

for K3PO4 -----------------> 3 K+ + PO4-3

0.1 M 3 x 0.1 0.1

(0.3 M) 0.1

m1 = 0.3 M

Z1 = +1

m2 = 0.1 M

Z2 = -3

ionic strength = 1/2 [m1z1^2 + m2 z^2 ]

= 1/2 [0.3 x 1^2 + 0.1 x (-3)^2]

= 1/2 (0.3 + 0.9)

= 0.6 M

why remaining options not correct:

the only confusion with option A ) H3PO4 weak electrolyte so it cannot completely dissociate into ions . so we cannot exactly write 3H+ and PO4-3. so ionic strength is very lesser

B) K+ and H2PO4-

z1 = z2 = 1 . so ionic strength lower

C) K2HPO4 ---------------> 2 K+ + HPO4-2

z1 = 1 , z2 = 2 . compared to K3PO4 ionic strength lower