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青州's Question
Chemistry
Posted 2 months ago
The concentration of ammonia in air in agricultural areas, measured as an hourly average, is often 
1
0
+
-
5
 parts per billion. Ammonia can react in aqueous phase according to the reactions below.
NH
3
 
(
g
)
 
⇆
 NH
3
 
(
aq
)
Henry's constant: K
_
H 
=
 
5
.
7
x
1
0
^
-
4
 mol L
^
-
1
 Pa
^
-
1
NH
3
 
(
aq
)
 
+
 H
2
O 
⇆
 NH
4
+
 
+
 OH
-
Equilibrium constant K
_
b 
=
 
1
.
7
8
x
1
0
^
-
5
 mol L
^
-
1
Assume that the concentration of ammonia in the atmosphere over an agricultural area is the typical 
1
0
 ppbv 
(
parts per billion by volume
)
,
 

at atmospheric pressure and a temperature of 
1
2
.
a
)
 Calculate the partial pressure of ammonia in air.
b
)
 Calculate the solubility of ammonia in water at pH 
6
.
5
 and state it in mol
/
I
.
 The solubility is calculated as the sum of NH
3
 
(
aq
)
 and NH
4
+
.
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Answer from Sia
Posted 2 months ago
Solution
1
Calculate the partial pressure of ammonia: To find the partial pressure of ammonia in air, we use the given concentration in ppbv and convert it to partial pressure using the total atmospheric pressure. The concentration of ammonia is given as 10 ppbv, which means 10 parts of ammonia per 1 billion parts of air. The standard atmospheric pressure is 101325 Pa
2
Conversion to partial pressure: The partial pressure of ammonia (PNH3P_{NH_3}) is calculated by multiplying the total atmospheric pressure by the ratio of the ammonia concentration to the total number of parts per billion
3
Mathematical expression for partial pressure: PNH3=10109×101325 PaP_{NH_3} = \frac{10}{10^9} \times 101325 \text{ Pa}
1 Answer
PNH3=1.01325×103 PaP_{NH_3} = 1.01325 \times 10^{-3} \text{ Pa}
Key Concept
Partial pressure is calculated by multiplying the concentration in ppbv by the total atmospheric pressure.
Explanation
The partial pressure of ammonia in the atmosphere is found by converting its concentration in parts per billion by volume to a pressure value using the standard atmospheric pressure.
4
Calculate the solubility of ammonia in water at pH 6.5: To calculate the solubility of ammonia in water, we need to consider the equilibrium reactions of ammonia in water and the given pH. The pH gives us the concentration of hydrogen ions ([H+][H^+]) in the solution
5
Determine the concentration of hydrogen ions: Using the pH value, we can calculate the concentration of hydrogen ions: [H+]=10pH[H^+] = 10^{-\text{pH}}
6
Calculate the concentration of hydroxide ions: The concentration of hydroxide ions ([OH][OH^-]) can be found using the water dissociation constant (Kw=1014 mol2 L2K_w = 10^{-14} \text{ mol}^2 \text{ L}^{-2}) and the concentration of hydrogen ions: [OH]=Kw[H+][OH^-] = \frac{K_w}{[H^+]}
7
Use Henry's law to find the concentration of NH3(aq): Henry's law states that the concentration of a gas in a liquid is directly proportional to its partial pressure above the liquid. The concentration of NH3(aq) is given by [NH3(aq)]=KH×PNH3[NH_3(aq)] = K_H \times P_{NH_3}
8
Calculate the concentration of NH4+: The equilibrium constant for the reaction of NH3 with water to form NH4+ and OH- is given by KbK_b. We can set up an equilibrium expression to find the concentration of NH4+: Kb=[NH4+][OH][NH3(aq)]K_b = \frac{[NH_4^+][OH^-]}{[NH_3(aq)]}
9
Solve for NH4+ concentration: Rearrange the equilibrium expression to solve for [NH4+][NH_4^+]: [NH4+]=Kb×[NH3(aq)][OH][NH_4^+] = \frac{K_b \times [NH_3(aq)]}{[OH^-]}
10
Sum the concentrations of NH3(aq) and NH4+: The solubility of ammonia in water is the sum of the concentrations of NH3(aq) and NH4+
2 Answer
The solubility of ammonia in water at pH 6.5 is the sum of [NH3(aq)][NH_3(aq)] and [NH4+][NH_4^+], which needs to be calculated using the given equilibrium constants and the pH value.
Key Concept
The solubility of ammonia in water is determined by the equilibrium of ammonia with water and the pH of the solution.
Explanation
To find the solubility of ammonia, we use Henry's law to calculate the concentration of dissolved ammonia and then use the equilibrium constant for the reaction of ammonia with water to find the concentration of ammonium ions. The sum of these concentrations gives the total solubility.

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