The solutions in three test tubes labeled A, B, and C all have the same pH. The
ID: 1055074 • Letter: T
Question
The solutions in three test tubes labeled A, B, and C all have the same pH. The test tubes are known to contain 1.0 times 10^-3 M HCl, 6.0 times 10^-3 M HCHO_2, and 4 times 10^-2 M C_6H_5NH_3^+. Describe a procedure for identifying the solutions. Consider the following titration curves. The solution in the buret is 0.1 M. The solution in the beaker has a volume of 50.0 mL. Answer the following questions. Is the titrating agent (solution in the buret) an acid or a base? Which curve shows the titration of the weakest base? What is the K_a of the conjugate acid of the base titrated in curve B? What is the molarity of the solution in the beaker for curve C? What is the pH at the equivalence point for curve A?Explanation / Answer
(a) The titrant is an acid as the final pH of the solution is acidic.
(b) C is the weakest base. As the initial pH is the least among the three.
(c) equivalence point is V = 50mL. half equivalence point is at V = 25 mL. and half equivalence point pH = pka
pka = 8.2. ka = 10^-8.2 = 6.31*10^-9
(d) ph of the starting point = 9.6, pOH = 14-9.6 = 4.4
[OH-] =10^-4.4 =3.98*10^-5
For C , pKa = pH at the half neutralization point = 7 . Ka = 10^-7
Kb of the base = Kw/Ka = 1*10^-14/ 10^-7 = 1*10^-7
B + H2O <==> BH^+ + OH-
Let us assume that inital concentration is M
Kb = x^2/M-x
x = 3.98*10^-5
as x is very small, M-x = M
M = x^2/Kb = (3.98*10^-5)^2/1*10^-7 = 1.58*10^-2 = 0.0158 M
(d) ph at the equivalence point is 4.
B BH^+ OH- initial M 0 0 change -x +x +x equilibrium M-x +x +x