A local neighborhood pond has a C_T = 1.0 mM, I = 0 and a pH = 8.3 in its surfac
ID: 1003783 • Letter: A
Question
A local neighborhood pond has a C_T = 1.0 mM, I = 0 and a pH = 8.3 in its surface water. Using pK'_a1 = 6.3 and pK'_a2 = 10.3, calculate the ratio of [CO^2-_3] to [HCO^-_3] and [H_2CO^*_3] to [HCO^-_3]. What is the concentration of [HCO^-_3]? Is this pond close to atmospheric equilibrium with current levels of atmospheric CO_2? Some agricultural runoff raises the NO^-_3 concentration in the pond to 15 mu M, adding the only significant N source to the pond. A dense algae bloom develops as a result. 1 times 10^-4 moles of C per liter of algal biomass is created and which follows the stoichiometry of the Redfield Ratio. What are the resultant changes in [HCO^-_3], [CO^2-_3] and pH? Is this system now at equilibrium with current atmospheric CO_2? What will tend to happen to alkalinity, pH and C_T as the surface water equilibrates with the atmosphere? How much NO^-_3 was consumed as a result of the bloom? How much NO^-_3 remains in solution? During this bloom, how many mu moles/L of O_2 were created? At the end of the bloom, the algae aggregates into clumps and sinks through the thermocline into a deeper water layer of the same volume as the surface layer. Heterotrophic organisms (bacteria and small animals) use this algal biomass as food and respire it. If there was initially 100 mu M O_2 in this deeper layer, what would happen as this biomass was respired, and what would the ending O_2 concentration be when all the biomass was finally consumed?Explanation / Answer
Solution:
a) From the data it is observed the schematic reaction as below,
H2CO3 + H2O <---> HCO3- + H3O+ pKa1=6.3
HCO3- + H2O <---> CO32- + H3O+ pKa2=10.3
by using hendersen-hasselbalch equation the ratio are computed for the above reaction scheme,
pH = pKa + log[HCO3-]/[CO32-]
log[HCO3-]/[CO32-] = pH - pK = 8.3 - 10.3 = -2
[HCO3-]/[CO32-] = e^(-2) = 0.1353
[CO32-] / [HCO3-] = 1/0.1353 = 7.389
The above ratio indicates that the given buffer can be prepared using 7.389 moles of NaHCO3 and 1 mol of Na2CO3
b) For the given scehemcatic reaction in pond CO2 is close to the atmospheric pressure at approximation.
c) due to agricultural run off influences there will be a change of 10% - 15% value is observed in all perspective.
d) No its not in equilibrium with atmospheric CO2 leads to high alkanity with increased basicity ( increase in pH value), creates an unfavourable condition for aquatic life in pond with decreased dissolved oxygen which results in dead of aquatic organisms by suffocation.
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