Chemistry question over acids?

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1 Answer
Feb 17, 2018

Consider,

#H_2A + H_2O rightleftharpoons HA^(-) + H^+#
puu.sh

where, #K_a = ([HA^-][H^+])/([H_2A])= 10^-3.67 = 2.14*10^-4#

Now, let's do algebra...

#x^2/(0.09M) = 2.14*10^-4#
#therefore x = [HA^-] = [H^+] = 1.92*10^-5M#

Moreover, consider,

#HA^- + H_2O rightleftharpoons A^(2-) + H^+#
puu.sh

where, #K_a = ([A^(2-)][H^+])/([HA^-])= 10^-6.11 = 7.76*10^-7#

Now, let's do even more algebra,

#(x * (1.92*10^-5 + x))/((1.92*10^-5 - x)) = 7.76*10^-7#

#therefore x = 7.20*10^-7M#

In an exam situation, you would probably have to use the quadratic equation here, but I'm not in an exam situation so I didn't. :)

Hence,

#[H^+] = 1.92*10^-5 + 7.20*10^-7 approx 2.0*10^-5M#

#(2.0*10^-5"mol")/"L" * (0.0750"L") approx 1.50*10^-6"mol"#

of hydronium ions exist in this particular solution of the diprotic acid.

These make for the best second-semester general chemistry exam questions...