Question

For a closed aqueous solution at pH 8.4, the bicarbonate species concentration is measured as 10-2.9 M. Calculate the concentrations of all carbonate system species, and the total dissolved inorganic carbon concentration in the system in moles/L, mg/L, and mg C/L.

Answer 1

Given, Concentration of HCO3- ion = 10^-2.9 M approx 10^-3 M at pH= 8.4.

Therefore, In 1 litre solution, [HCO3-] = 10^-3 × 1 mol

= 10^-3 mol

Here, H2CO3 (pka1) HCO3- (pka2) CO32-

pka2= 10.32 (from data table of pka value of different ions )

From Henderson Hassel back Equation:

pH= pka2 + log[CO32-]/[HCO3-]

8.4= 10.32+ log[CO32-]/10^-3

-1.92= log[CO32-]/10^-3

[CO32-]= 10^-1.92 × 10^-2.9 M

= 10^-4.82 M

Now, molar weight of CO32- = 60 grams

Molar weight of HCO3- = 61 grams

Therefore, moles of [HCO3-] + [CO32-] in 1 litre solution

= 10^-2.9 M × 1 litre + 10^-4.82 M × 1 litre.

= Total dissolved inorganic carbon content

(As both [HCO3-] and [CO32-] have one C atom each)

= (10^-2.9 + 10^-4.82 ) moles inorganic carbon content.

= (10^-2.9 + 10^-4.82) × 12× 1000 mg/L

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