Concept explainers
(a)
Interpretation:
Species among H2CO3, HCO-3, CO2-3 having highest concentration at pH = 8.5 has to be calculated.
Concept introduction:
pH:
pH is a scale used to specify the acidity or basicity a solution. It ranges from 0−14. pH 7.0 is considered as neutral solution, pH more than 7.0 is taken as basic solution whereas pH less than 7.0 is considered as acidic solution (at 25οC). It is the measurement of activity of free H+ and OH- in solution.
pKa:
pKa is the negative log of acid dissociation constant that determines the strength of the acid. Lower the pKa value stronger is the acid.
pKa1 is the 1st dissociation constant for the acid and pKa2 is the 2nd dissociation constant of the acid.
(a)
Answer to Problem 19.142P
Among all the three species, HCO-3 is more at pH = 8.5.
Explanation of Solution
The equation considered as,
H2CO3⇄H+ + HCO-3 ...(Ka1)
HCO-3⇄ H+ + CO2-3 ...(Ka2)
pKa = −logKa
Now, for the 1st equation,
H2CO3⇄H+ + HCO-3 ...(Ka1)
Ka1 = 4.5×10−7
pKa1 = −log(4.5×10−7)
pKa1 = 6.4368
Now, for the 2nd equation,
HCO-3⇄ H+ + CO2-3 ...(Ka2)
Ka2 = 4.7×10−11
pKa2 = −log(4.7×10−11)
pKa2 = 10.3279
From the above data it is shown that, pKa2 > pH but pKa1 < pH. (pH = 8.5 given)
If pKa1 is less that means Ka1 value is more which means the 1st dissociation occurs rapidly and hence HCO-3 will be more in medium (as the pH is basic so the conjugate base is more in concentration.)
Again, pKa2 is more that means Ka2 is less and hence 2nd dissociation is slower in rate. Hence in medium the reactant i.e. HCO-3 will be more.
Hence among all the three species HCO-3 is more at pH = 8.5.
(b)
Interpretation:
At pH = 8.5 the concentration ratio of [HCO-3][H2CO3] and [CO2-3][HCO-3] has to be calculated.
Concept introduction:
pH:
pH is a scale used to specify the acidity or basicity a solution. It ranges from 0−14. pH 7.0 is considered as neutral solution, pH more than 7.0 is taken as basic solution whereas pH less than 7.0 is considered as acidic solution (at 25οC). It is the measurement of activity of free H+ and OH- in solution.
pKa:
pKa is the negative log of acid dissociation constant that determines the strength of the acid. Lower the pKa value stronger is the acid.
pKa1 is the 1st dissociation constant for the acid and pKa2 is the 2nd dissociation constant of the acid.
Henderson-Hasselbalch Equation:
This equation is used to determine the pH of a buffer solution. Ka is the given quantity which is acid dissociation constant. It is done for a given concentration of acid and its conjugate base.
pH= pKa + log [salt][acid]
(b)
Answer to Problem 19.142P
The value of concentration ratios of [HCO-3][H2CO3] and [CO2-3][HCO-3] are respectively 1.4×102 and 1.48×10−2 .
Explanation of Solution
The equation considered as,
H2CO3⇄H+ + HCO-3 ...(Ka1)
HCO-3⇄ H+ + CO2-3 ...(Ka2)
pKa = −logKa
For the 1st equation,
H2CO3⇄H+ + HCO-3 ...(Ka1)
Thus the pKa is calculated as,
Ka1 = 4.5×10−7
pKa1 = −log(4.5×10−7)
pKa1 = 6.4368
Thus the pH is calculated as,
pH= pKa + log [salt][acid]
pH= pKa1 + log [HCO-3][H2CO3]8.5 = 6.35 + log [HCO-3][H2CO3]
The value of concentration ratios of [HCO-3][H2CO3]= 1.4×102
For the 2nd equation,
HCO-3⇄ H+ + CO2-3 ...(Ka2)
Thus the pKa is calculated as,
Ka2 = 4.7×10−11
pKa2 = −log(4.7×10−11)
pKa2= 10.33
Thus the pH is calculated as,
pH= pKa2 + log [CO2-3][HCO-3]8.5 = 10.33 + log [CO2-3][HCO-3] [CO2-3][HCO-3]= 1.48×10−2
The value of concentration ratios of [CO2-3][HCO-3]= 1.48×10−2
Thus the value of concentration ratios of [HCO-3][H2CO3] and [CO2-3][HCO-3] are respectively 1.4×102 and 1.48×10−2 .
(c)
Interpretation:
Reason for lower pH at deep sea level has to be interpreted.
Concept introduction:
pH:
pH is a scale used to specify the acidity or basicity a solution. It ranges from 0−14. pH 7.0 is considered as neutral solution, pH more than 7.0 is taken as basic solution whereas pH less than 7.0 is considered as acidic solution (at 25οC). It is the measurement of activity of free H+ and OH- in solution.
Photosynthesis:
The process of photosynthesis occurs in plants for their survival. In this process carbon dioxide is absorbed from nature and oxygen is released to nature by plants. Sunlight is the main important component for photosynthesis to occur. Here glucose is formed.
6 CO2 + 6 H2O sunlight⇄ C6H12O6 + 6O2
Respiration:
Respiration is a biochemical process in which in the cells of organisms energy is evolved due to reaction between oxygen and glucose and by this process carbon dioxide is released along with water and ATP
C6H12O6 + 6O2 ⇄ 6 CO2 + 6 H2O + ATP
(c)
Explanation of Solution
The acidity of the sea water is due to presence of dissolved carbon dioxide in the water.
On the upper sea level there are plants and also on upper level of sea sunlight can enter. As a result of that on the upper level of sea photosynthesis occurs and according to the process of photosynthesis carbon dioxide is taken up by plants. This results a decrease in the dissolved carbon dioxide concentration in the upper sea level.
But still due to the respiration by the sea animals and the plants at upper level there will be some amount of dissolved carbon dioxide in the upper level. So sea water at upper level is acidic but less in amount.
However, in deep sea level, pH = 7.5 that means water is more acidic here. In deep sea level pressure is very high and sunlight cannot enter and thus plants cannot survive there but some animals only can survive there. Hence photosynthesis does not occur there only respiration occurs. As a result of that the concentration of dissolved carbon dioxide is more in the deep-sea level than in the upper level. So the acidity here is more.
Hence, deep sea water has a higher acidity the value of pH = 7.5.
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Chapter 19 Solutions
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