Question

Consider the solutions 0.010 M HNO₃, 0.10 M HCl, and 0.010 M HNO₂ . Which solution has the highest pH? the lowest pH? [Please] Explain.

Answer 1

First calculate Ph of all the given acids and the compare, HNO3 and HCl are strong acids and hence 100% ionize. Whereas HNO2 is weak acid and not fully ionize.

pH of 0.010 M HNO3

[H+] = 0.010M

pH = -log[H+]

pH = -log0.01

pH = 2

pH of 0.10 M HCl

[H+] = 0.10M

pH = -log0.10

pH = 1

pH of  0.010 M HNO2 ie the value of concentration "c" Ka for HNO2 is 4 x 10^-4

pH of weak acids can be given as:

pH = 1/2[pKa - logc] ------------------(1)

where in this case pKa of HNO2 = -logKa = -log4 x 10^-4

pKa = - (-3.4)

pKa = 3.4

put all these values in above equation (1):

pH = 1/2[3.4 - log0.010]

pH = 1/2[3.4 –(-2)]

pH = 1/2[3.4 + 2)]

pH = ½[5.4]

pH = 2.7

So, highest pH by 0.010M HNO2 and lowest pH by 0.10M HCl