Shanxian Chemical
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Calculate Hydrogen Ion Concentration
On the Calculation of Hydrogen Ion Concentration
If you want to understand the principle of chemistry, the calculation of hydrogen ion concentration is the key. The method is not the same, and it should be carefully observed and carefully pursued.
1. According to the ionization constant of the acid, if
is a strong acid, it is almost completely ionized in water. For example, hydrochloric acid ($HCl $), let the amount and concentration of its substance be $c $, then the hydrogen ion concentration $[H ^ {+}] = c $. If the strong acid is covered in water, $HCl\ rightarrow H ^ {+} + Cl ^ {-} $, the ionization is complete, and all hydrogen ions come from this.
If it is a weak acid, take acetic acid ($CH_ {3} COOH $) as an example, when the ionization constant is $K_ {a} $. Acetic acid in water has an ionization equilibrium of $CH_ {3} COOH\ rightleftharpoons CH_ {3} COO ^ {-} + H ^ {+} $, let the initial acetic acid concentration be $c_ {0} $, and when the equilibrium is reached, the hydrogen ion concentration is $x $, the acetate ion concentration is also $x $, and the acetic acid equilibrium concentration is $c_ {0} -x $. According to the ionization constant expression $K_ {a} =\ frac {[CH_ {3} COO ^ {-}] [H ^{+}]}{[ CH_ {3} COOH]} $, that is, $K_ {a} =\ frac {x\ cdot x} {c_ {0} -x} $. If $c_ {0} $is large, $K_ {a} $is small, and $x $is very small relative to $c_ {0} $, it can be approximated that $c_ {0} -x\ approx c_ {0} $, then $x =\ sqrt {K_ {a} c_ {0}} $, that is, the hydrogen ion concentration $[H ^{+}]=\ sqrt {K_ {a} c_ {0}} $.
Second, from the value of $pH $, the negative logarithm of the hydrogen ion concentration is also, that is, $pH = -\ lg [H ^{+}]$。 If the pH value of the solution is known, and the hydrogen ion concentration is desired, it can be inverted. If pH = 3 $, then $-\ lg [H ^ {+}] = 3 $, that is, $\ lg [H ^{+}]= - 3 $. From the logarithmic property, we can get $[H ^ {+}] = 10 ^ {-3} mol/L $.
Third, find
in a salt solution. For strong acids and weak bases, such as ammonium chloride ($NH_ {4} Cl $), its hydrolysis makes the solution acidic. $NH_ {4} ^ {+} + H_ {2} O\ rightleftharpoons NH_ {3}\ cdot H_ {2} O + H ^{+}$ 。 Let the ammonium chloride concentration be $c $and the hydrolysis constant be $K_ {h} $. The hydrolysis constant is related to the corresponding weak base ionization constant $K_ {b} $K_ {h} =\ frac {K_ {w}} {K_ {b}} $ ($K_ {w} $is the ionic product constant of water). When equilibrium is reached, let the hydrogen ion concentration be $x $, $NH_ {3}\ cdot H_ {2} O $concentration is also $x $, $NH_ {4} ^ {+} $equilibrium concentration is $c - x $, by the hydrolysis constant expression $K_ {h} =\ frac {[NH_ {3}\ cdot H_ {2} O] [H ^{+}]}{[ NH_ {4 }^{+}]}$， that is, $K_ {h} =\ frac {x\ cdot x} {c - x} $. If $c $is larger, $K_ {h} $is smaller, approximating $c - x\ approx c $, then $x =\ sqrt {K_ {h} c} $, that is, $[H ^{+}]=\ sqrt {K_ {h} c} $.
In short, the concentration of hydrogen ions can be obtained by considering the properties of the solution, or according to the strength of the acid, or according to the value of $pH $, or the hydrolysis of salts.
If you want to understand the principle of chemistry, the calculation of hydrogen ion concentration is the key. The method is not the same, and it should be carefully observed and carefully pursued.
1. According to the ionization constant of the acid, if
is a strong acid, it is almost completely ionized in water. For example, hydrochloric acid ($HCl $), let the amount and concentration of its substance be $c $, then the hydrogen ion concentration $[H ^ {+}] = c $. If the strong acid is covered in water, $HCl\ rightarrow H ^ {+} + Cl ^ {-} $, the ionization is complete, and all hydrogen ions come from this.
If it is a weak acid, take acetic acid ($CH_ {3} COOH $) as an example, when the ionization constant is $K_ {a} $. Acetic acid in water has an ionization equilibrium of $CH_ {3} COOH\ rightleftharpoons CH_ {3} COO ^ {-} + H ^ {+} $, let the initial acetic acid concentration be $c_ {0} $, and when the equilibrium is reached, the hydrogen ion concentration is $x $, the acetate ion concentration is also $x $, and the acetic acid equilibrium concentration is $c_ {0} -x $. According to the ionization constant expression $K_ {a} =\ frac {[CH_ {3} COO ^ {-}] [H ^{+}]}{[ CH_ {3} COOH]} $, that is, $K_ {a} =\ frac {x\ cdot x} {c_ {0} -x} $. If $c_ {0} $is large, $K_ {a} $is small, and $x $is very small relative to $c_ {0} $, it can be approximated that $c_ {0} -x\ approx c_ {0} $, then $x =\ sqrt {K_ {a} c_ {0}} $, that is, the hydrogen ion concentration $[H ^{+}]=\ sqrt {K_ {a} c_ {0}} $.
Second, from the value of $pH $, the negative logarithm of the hydrogen ion concentration is also, that is, $pH = -\ lg [H ^{+}]$。 If the pH value of the solution is known, and the hydrogen ion concentration is desired, it can be inverted. If pH = 3 $, then $-\ lg [H ^ {+}] = 3 $, that is, $\ lg [H ^{+}]= - 3 $. From the logarithmic property, we can get $[H ^ {+}] = 10 ^ {-3} mol/L $.
Third, find
in a salt solution. For strong acids and weak bases, such as ammonium chloride ($NH_ {4} Cl $), its hydrolysis makes the solution acidic. $NH_ {4} ^ {+} + H_ {2} O\ rightleftharpoons NH_ {3}\ cdot H_ {2} O + H ^{+}$ 。 Let the ammonium chloride concentration be $c $and the hydrolysis constant be $K_ {h} $. The hydrolysis constant is related to the corresponding weak base ionization constant $K_ {b} $K_ {h} =\ frac {K_ {w}} {K_ {b}} $ ($K_ {w} $is the ionic product constant of water). When equilibrium is reached, let the hydrogen ion concentration be $x $, $NH_ {3}\ cdot H_ {2} O $concentration is also $x $, $NH_ {4} ^ {+} $equilibrium concentration is $c - x $, by the hydrolysis constant expression $K_ {h} =\ frac {[NH_ {3}\ cdot H_ {2} O] [H ^{+}]}{[ NH_ {4 }^{+}]}$， that is, $K_ {h} =\ frac {x\ cdot x} {c - x} $. If $c $is larger, $K_ {h} $is smaller, approximating $c - x\ approx c $, then $x =\ sqrt {K_ {h} c} $, that is, $[H ^{+}]=\ sqrt {K_ {h} c} $.
In short, the concentration of hydrogen ions can be obtained by considering the properties of the solution, or according to the strength of the acid, or according to the value of $pH $, or the hydrolysis of salts.
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