Question

Bicarbonate (HCO₃^-) is a common water quality constituent. Bicarbonate dissociates as a weak acid to give carbonate (CO₃^2-) as follows:

HCO₃^- = CO₃^2- + H⁺ pK_a = 10.33

Determine whether HCO₃^-, or CO₃^2-, is predominant in natural waters at pH = 7.90.

I got (HCO_3)^- ; I would just like to double check!

Answer 1

When Camban dioxide react with water to produce weak acid Carbonic acid CH₂CO3). which feather dissociates to give becoabonate and then carbonate CO₂ + H₂O - H₂CO3 Horog - *** + HCO3 pH in cases Ho -> com +44 Bicaabanate and carbonate acts as buffet is water HCo c o + H+ weak acid conjugate Given pH d natural waters = 7.90 pka of HCO = 10.33 pH of buffer is calculated wing Henderson. Hasselbalch equation : pH = pk + log [conjugate base) I weak acid

is geophanged as blows the above epuakion Ka - [co 2- [H] [hcoj] pH = 7.90 = -log [ht] [++] = 107.90 [H+] = 1,253 x1024 pka = 10:33 -pka ka = 10 - -10.33 Ka = 10 Ka = 4.677x10 " substitute, [H+J & Ka value in equation 6 , we 4.677 X10 [co 2- ] 4.677 X10 [Heg] 1. 258 x108M [cool] [Hoog -0.0037

The ratio of [co2m] / [420 7 = 0.0037 which show that [Heog] = 1 and [CO32- ] = 0.0037. : HCO is predominant in natural teaters at pH = 7.90 [co 2-] 3.7 [+cog ] 1000 CO + H₂O predominant in - natural Helg roh is waters at pH = 7.90

HCO₃^- is predominant in natural waters at pH = 7.90

The ratio of [CO₃^2-]/[HCO₃^-​​​​​​] is 3.7/1000