Equal quantities of 0.010M solutions of an acid HA and a base B are mixed. The p
ID: 821106 • Letter: E
Question
Equal quantities of 0.010M solutions of an acid HA and a base B are mixed. The pH of the resulting solution is 9.4. If K a for HA is 6.0 times 10?5, what is the value of the equilibrium constant for the reaction between HA and B? Need Help With C & D. Write the equilibrium equation for the reaction between HA and B. Write equilibrium-constant expression for the reaction between HA and B. [HB +] [A-] / [HA] [B] If K a for HA is 6.0 times 10?5, what is the value of the equilibrium constant for the reaction between HA and B ? What is the value of K b for B ?Explanation / Answer
First, realize that this is an aqueous solution you're working with. Second, you have to understand the concept of "solvent leveling" - which, when the solvent is water, basically says that there are no stronger bases present in aqueous solution than OH? and no stronger acids present than H?O?. The reason isn't difficult to understand by considering an example. The NH?? ion is a stronger base than OH?, but in water it completely reacts with water to give OH? plus NH?: NH??(aq) + H?O ? OH?(aq) + NH?(aq). So, what does this have to do with your problem? If you mix 0.010 M solutions of an acid HA and a base B, and B is a stronger base than OH?, it will just react with water to give OH?. Therefore the strongest base you could be combining with HA is hydroxide. Since you are told that you're mixing equal volumes of HA and B solutions of equal concentration, the most basic possible solution you could have would be one in which all the HA is converted to A?, because that is just what would happen if B is hydroxide. Since you are adding two 0.01 M solutions, the final concentration of A? would be 0.005 M (the volume doubles).
Then consider the equilibrium
. . A?(aq) + H?O(aq) ? HA + OH?(aq)
0.005 - x . . . . . . . . . . . . x . . . x ? concentrations at equilibrium
The Kb for A? is given by
Kb = Kw/K?(HA) = 10?
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