Respuesta :
Unpolluted rainwater has a pH of 5.59.
Steps
The following sentence can be used to illustrate how carbon dioxide dissolves in water:
H2O + CO2 --> H2CO3
The amount of carbon dioxide produced must also be expressed in order to determine the amount of carbonic acid created.
Knowing the amount of carbon dioxide (the limiting reactant) is adequate because water will be produced in excess in this reaction due to the low concentration of carbon dioxide in the air.
The ideal gas law can be used to determine the quantity of CO 2 because it is a gas. Let's take 100 mL of water at random (any amount would be suitable).
VH2O = 100mL = 0.1L
VCO2 = 88mL = 0.000088m^3
pV = nRT
n = 3.597×10^−3mol
Let's determine the concentration of the maximum dissolved CO2 using the amount of water (the solute) as our input. Since there is little volume change during dissolution, the water volume can be utilised.
[CO2]maximum = n / Vwater
[CO2]maximum = 3.597×10 ^−2M
The concentration of CO2 in the air is quite low, hence this maximum concentration won't exist. The real concentration of CO 2 in rainfall can be calculated by taking into consideration the volume% of CO 2 in air.
[CO2] = [CO2]maximum × V/V CO2
[CO2] = 1.44×10^−5M
Let's now use the equation below to represent the K a constant of H 2CO3. Looking at the first equation above, we can see that the stoichiometric ratios of CO 2 and H 2CO 3 are the same, hence all 1.44 x 10-5 M of carbon dioxide will dissolve into 1.44 x 10-5 M of carbonic acid during the dissolution of carbon dioxide. As a result, we can use the CO2 concentration estimated as the H2CO3 concentration.
H2CO3 + H20--> HCO3 + H3O
Ka = [H+][HCO3−] / H2CO3
Since the proton and the hydrocarbonate ion have the identical stoichiometric number, we may simply represent them by the letter x.
[tex]4.5 \times 10^{-7} = \dfrac{x^2}{1.44 \times 10^{-5}}[/tex]
x= [H+] = 2.55 x 10^-6
PH = -log [H+]
PH =5.59
Unpolluted rainwater has a pH of 5.59.
learn more about pH here
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