What stoichiometric concentration of the indicated substance is required to obtain an aqueous solution with the pH value shown below?
a)aniline, C6H5NH2, for pH= 9.05
b)NH4Cl for pH= 5.22
a) Aniline is a base.
pH = 9.05; pOH = 4.95; OH^- = 1.12E-5M
......C6H5NH2 + HOH ==> C6H5NH3^+ + OH^-
initial..x................0..........0
change..-1.12E-5......1.12E-5..1.12E-5
equil..x-1.12E-5....1.12E-5...1.12E-5
Kb = (C6H5NH3^+)(OH^-)/(C6H5NH2)
Substitute into the Kb expression, look up Kb for aniline, solve for x = M aniline for pH of 9.05
b) The pH is determined by the hydrolysis of the salt. pH = 5.22; (H^+) = 6.03E-6
........NH4^+ + H2O ==> H3O^+ + NH3
initial...x...............0.......0
change..-6.03E-6.....6.03E-6..6.03E-6
equil..x-6.03E-6......6.03E-6..6.03E-6
Ka for NH4^+ = (Kw/Kb for NH3) = (NH3)(H3O^+)/(NH4^+)
Substitute into the Ka expression from above and solve for x = (NH4+) = (NH4Cl)
initial.
When I do a) I get an answer of 0.298M, however, it says I'm wrong. b) worked perfectly
I obtained 0.299 but you have used 4.2E-10 for Kb which is just two significant figures. I would round the 0.298 you obtained to 0.30 and submit that. I would appreciate letting me know if this takes care of the problem. This post is getting buried so just make a new question to DrBob222 and let me know. Thanks.
To determine the stoichiometric concentration of a substance required to obtain an aqueous solution with a specific pH value, you need to consider the acid-base properties of the substance. Specifically, you need to determine if the substance acts as an acid or a base in water.
a) In the case of aniline, C6H5NH2, it is a weak base that partially dissociates in water according to the following equilibrium reaction:
C6H5NH2 + H2O ⇌ C6H5NH3+ + OH-
To obtain an aqueous solution with a pH value of 9.05, you need to calculate the concentration of hydroxide ions (OH-) in the solution. The concentration of OH- can be determined using the equation:
pOH = -log[OH-]
Since you have the pH value of 9.05, you can find the pOH value by subtracting the pH from 14:
pOH = 14 - 9.05 = 4.95
Next, convert the pOH value back to the concentration of hydroxide ions (OH-) using the equation:
[OH-] = 10^(-pOH)
[OH-] = 10^(-4.95)
Now, to determine the stoichiometric concentration of aniline required, you need to consider the stoichiometry of the reaction. Since aniline is a weak base, it reacts with hydroxide ions in a 1:1 ratio. Therefore, the concentration of aniline will be equal to the concentration of hydroxide ions:
[C6H5NH2] = [OH-] = 10^(-4.95)
b) Similarly, for NH4Cl, you need to consider its acid-base properties. NH4Cl is a salt that dissociates in water to form NH4+ and Cl- ions. The NH4+ ion can act as a weak acid, while the Cl- ion is a spectator ion that does not participate in the pH calculation.
NH4+ + H2O ⇌ NH3 + H3O+
To obtain an aqueous solution with a pH value of 5.22, you need to calculate the concentration of hydronium ions (H3O+) in the solution. The concentration of H3O+ can be determined using the equation:
pH = -log[H3O+]
Since you have the pH value of 5.22, the concentration of H3O+ can be calculated as follows:
[H3O+] = 10^(-pH)
[H3O+] = 10^(-5.22)
Since NH4+ acts as a weak acid and reacts with water to form H3O+, you can determine the stoichiometric concentration of NH4+ required, which will be equal to the concentration of H3O+:
[NH4+] = [H3O+] = 10^(-5.22)
These calculations will give you the stoichiometric concentrations of aniline (C6H5NH2) and NH4Cl required to obtain aqueous solutions with the specified pH values.