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...

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

We know that Ka x Kb = 1.0x10-14

HNO2 ( Ka=4.5 x10-4)

HCN (Ka= 4.9 x10-10)

CH3NH2 (Kb=4.4 x10-4) ---> Ka = ( 1.0x10-14 )/Kb = ( 1.0x10-14 ) / (4.4 x10-4) = 2.27x10-11

HONH2 (Kb=1.1 x10-8) ---->  Ka = ( 1.0x10-14 )/Kb = ( 1.0x10-14 ) / (1.1 x10-8) = 9.09x10-7

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

= pH decreases

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

Order of Ka : 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

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