The ocean has a natural carbonate buffer system, which helps to regulate the pH
ID: 962208 • Letter: T
Question
The ocean has a natural carbonate buffer system, which helps to regulate the pH of seawater. a) Write the two successive dissociation reactions for carbonic acid (H2CO3), and their associated acid dissociation constants (Ka expressions). b) What is the pH of a buffer that is created by combining 65.0 ml of 1.00 M H2CO3 and 35.0 ml of 0.800 M NaHCO3? (assume the second dissociation is negligible and dont forget to account for dilution). c)What would the pH of the solution be after 1.5 ml of 2.5 M HCl is added?
Explanation / Answer
a)
H2CO3 <-------------------> HCO3- + H+
Ka1 = [HCO3-][H+] / [H2CO3]
HCO3- <-------------------> CO32- + H+
Ka2 = [CO32-][H+] / [HCO3-]
b)
millimoles of H2CO3 = 65 x 1 = 65
millimoles of NaHCO3 = 35 x 0.8 = 28
pH = pKa + log [NaHCO3 / H2CO3]
= 6.35 + log [28 / 65]
= 5.98
pH = 5.98
c)
millimoles of HCl = 1.5 x 2.5 = 3.75
pH = pKa + log [salt - C / acid + C]
= 6.35 + log [28 - 3.75 / 65 + 3.75]
= 5.90
pH = 5.90
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