Question

HNO2 ( Ka=4.5 x 10^-4)

HCN (Ka= 4.9 x 10^-10)

CH3NH2 (Kb=4.4 x 10^-4)

HONH2 (Kb=1.1 x 10^-8)

Use this data to rank the following solutions in order of increasing pH. In other words, select a '1' next to the solution that will have the lowest pH, a '2' next to the solution that will have the next lowest pH, and so on..

RANK THESE ONES:

0.1 M NaBr

0.1 M CH3NH3Cl

0.1 M KNO2

0.1 M HONH3Br

Answer 1

We know that K_a x K_b = 1.0x10^-14

HNO₂ ( K_a=4.5 x10^-4)

HCN (K_a= 4.9 x10^-10)

CH₃NH₂ (K_b=4.4 x10^-4) ---> K_a = ( 1.0x10^-14 )/K_b = ( 1.0x10^-14 ) / (4.4 x10^-4) = 2.27x10^-11

HONH₂ (K_b=1.1 x10^-8) ---->  K_a = ( 1.0x10^-14 )/K_b = ( 1.0x10^-14 ) / (1.1 x10^-8) = 9.09x10^-7

Also we know that As K_a increases [H⁺] concentration increases log [H⁺] increases ---> - log [H⁺] decreases

= pH decreases

So high value of K_a has low pH ( The given concentrations are same for all)

Order of K_a : HNO₂ > HONH₂ > HCN > CH₃NH₂

Therefore the order of pH is : HNO₂ < HONH₂ < HCN < CH₃NH₂

   So the order of given compounds is reversed(since the salts are taken)

KNO₂ > HONH3Br > NaBr > CH₃NH3Br