Solutions Question 388
Question: The partial pressure of $ CO_2 $ in air saturated with water vapours at $ 25{}^\circ C $ and 1.00 atm is $ 3.04\times {10^{-4}} $ atm. Henry’s constant for $ CO_2 $ in water is $ 2.3\times {10^{-2}},mol,{L^{-1}}at{m^{-1}} $ and for carbonic acid $ pK_{a} $ is 6.37. The pH of normal ( $ CO_2 $ mixed) rain water will be
Options:
A) 7.42
B) 6.37
C) 6.99
D) 5.76
Show Answer
Answer:
Correct Answer: D
Solution:
Solubility $ =K,{p_{CO_2}} $
$ =2.3\times {10^{-2}}\times 3.04\times {10^{-4}} $
$ =6.992\times {10^{-6}}mol,{L^{-1}} $
$ H_2CO_3{H^{+}}+HCO_3^{-}\Rightarrow [{H^{+}}]=\sqrt{K_{a}.,S} $
$ pH=\frac{1}{2}[pK_{a}-\log S] $
$ =\frac{1}{2}[ 6.37-\log 6.992\times {10^{-6}} ] $
$ =\frac{1}{2}[6.37+5.16],\Rightarrow pH=5.76 $
