What is the #"pH"# of a #"1.5-M"# solution of ammonia? The dissociation constant of ammonia at #25.0^@"C"# is #1.80 * 10^-5#.
1 Answer
Explanation:
As you know, ammonia acts as a weak base in aqueous solution, so right from the start, you should expect the
#"NH"_ (3(aq)) + "H"_ 2"O"_ ((l)) rightleftharpoons "NH"_ (4(aq))^(+) + "OH"_ ((aq))^(-)#
The ratio that exists between the equilibrium concentrations of the ammonium cations and of the hydroxide anions and the equilibrium concentration of ammonia is given by the base dissociation constant,
#K_b = (["NH"_4^(+)] * ["OH"^(-)])/(["NH"_3])#
Now, ammonia will only partially ionize to produce ammonium cations and hydroxide anions. If you take
#["NH"_4^(+)] = ["OH"^(-)] = x quad "M"# This happens because every mole of ammonia that ionizes produces
#1# mole of ammonium cations and#1# mole of hydroxide anions.So if
#x# #"M"# ionizes, you can expect the solution to contain#x# #"M"# of the two ions.
The solution will also contain
#["NH"_3] = (1.5 - x) quad "M"# When
#x# #"M"# ionizes, the initial concentration of ammonia will decrease by#x# #"M"# .
This means that the expression of the base dissociation constant will now take the form
#K_b = (x * x)/(1.5 - x)#
which is equal to
#1.80 * 10^(-5) = x^2/(1.5 - x)#
Notice that the value of the base dissociation constant is significantly smaller than the initial concentration of the base. This tells you that you can use the approximation
#1.5 -x ~~ 1.5#
because the concentration of ammonia that ionizes will be significantly lower than the initial concentration of the base, i.e. the ionization equilibrium will lie to the left.
You now have
#1.80 * 10^(-5) = x^2/1.5#
Rearrange and solve for
#x = sqrt(1.5 * 1.80 * 10^(-5)) = 0.005196#
Since
#["OH"^(-)] = "0.005196 M"#
Now, an aqueous solution at
#"pH + pOH = 14"#
Since
#"pOH" = - log(["OH"^(-)])#
you can say that the
#"pH" = 14 + log(["OH"^(-)])#
Plug in your value to find
#"pH" = 14 + log(0.005196) = color(darkgreen)(ul(color(black)(11.72)))#
The answer is rounded to two decimal places because you have two sig figs for the molarity of the solution.