Question
Question: Which one is correct for \({{H}_{2}}O\) at \({{25}^{\circ }}C\)? (a) Ionic product of water, \({{K...
Which one is correct for H2O at 25∘C?
(a) Ionic product of water, Kw=10−14
(b) Equilibrium constant for dissociation of water Kc=1.8×10−16
(c) Autoprotolysis constant of water, KAP=3.2×10−18
(d) On heating Kw increases with temperature
Solution
We should know that temperature plays an important role in determining the values of different constants of a liquid thus always carefully check and calculate the values according to the given temperature. Here we are provided water at 25∘C.
Complete answer:
H2O is one of the most important substances for the survival of living beings on earth and hence it is necessary to study and learn its properties.
We know that ionic product of water is: Kw=[H+][OH−]
Therefore Kw of water is dependent on the concentration of H+ and OH− ions in the solution.
Now, let us go through the options one by one:-
(a) Ionic product of water, Kw=10−14 :-
Since H2O at 25∘C, the active mass of H+ and OH− ions is approximately equal to 10−7 units. So Kw of water will be : Kw=[H+][OH−]
⇒10−7×10−7=10−14
(b) Equilibrium constant for dissociation of water Kc=1.8×10−16 :-
For a reaction AxBy→xA+yB, Dissociation constant (Kc) = [AxBy][A]x[B]y
So for H2O→2H++OH−, Dissociation constant (Kc) = [H2O][H+]2[OH−]
Using molar properties of H2Oat25∘Cand further calculations it is observed that Kc for water is 1.8×10−16
(c) Autoprotolysis constant of water, KAP=3.2×10−18:-
Through several experimental observation, scientists have observed that at 25∘C, the KAP of water is 3.2×10−18.
(d) On heating Kw increases with temperature:-
As we increase the temperature, the amount of available energy required to cross the activation energy barrier also increases. Thus a large number of H+ and OH− ions will be observed. Since we already discussed above that Kw of water is dependent on the concentration of H+ and OH− ions so on heating Kw increases with temperature.
-Hence the correct answers are (a), (b), (c) and (d).
Note:
Water (25∘C) is the most abundant and important substance on the planet earth, thus it is important that we learn and remember its properties.
-Certain values of important constants are calculated in scientific laboratories after various measurements. Those values are provided to us by the scientists and we must try to learn these important values in chemistry.