The carbonate system is the most important contributor to the acid-base chemistr

Question

The carbonate system is the most important contributor to the acid-base chemistry of seawater `but the carbonate system is complicated because carbon is non-conservative and so varies in its concentration from place to place. Borate is conservative and maintains a constant ratio to salinity. Let’s calculate the equilibrium concentrations of boric acid (B(OH)3) and the borate anion (B(OH)4-) in pH 8 seawater, given a total boron concentration (B(OH)3 + B(OH)4-) = 4.2 x 10-4 mol L-1 and pK = 8.6. Assume ideal conditions, that the activity coefficients are one. The equilibrium reaction is:

B(OH)3 + H2O ßà B(OH)4- + H+

Explanation / Answer

we have a buffer i,e weak acid B(OH)3 and its conjugate base B(OH)4-

we use Henderson eq of buffer

pH = pka + log [ conjugate base] /[acid]

8 = 8.6 + log [B(OH)4-] / [B(OH)3]

[B(OH)4-] = 0.25 [B(OH)3] .............(1)

we were given [B(OH)3] + [B(OH)4-] = 0.00042 .............(2)

solvig (1) and (2)

we get [B(OH)3] = 3.36 x 10^- 4 M

[B(OH4)-] = 8.4 x 10^- 5 M

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