Can someone please help me by explaining and correcting me?
1) A 0.057M solution of a weak acid has a pH of 4.93. Determine the ionization constant, Ka, of the acid.
a) 2.1 x 10^-4
b) 2.4 x 10^-9
c) 4.1 x 10^-8
d) 2.8 x 10^-7
e) 1.4 x 10^-10
-- To be honest, I skipped this. I know that the Ka = ((H3O^+)(A^-))/(HA), but I didn't know how to find (A^-). Am I approaching this question wrong? Is there a step I missed or can this be solved another way? Perhaps it has something to do with an ICE chart?
2) For the following compound, predict whether the solution is acidic, basic, or neutral and why: NH4Cl.
a) acidic, NH4Cl is the salt of a weak base
b) neutral because there is no hydrolysis
c) acidic, NH4Cl is a strong acid
d) basic, NH4Cl is the salt of a weak acid
e) basic, NH4Cl is a weak base
-- I picked D. I think I may have chosen wrong. I think it dissociates into NH4^+ and Cl^- making an acidic solution... but I'm not so sure
3)
For a reaction where rate = k (A)(B)^2, what factor will NOT change k?
a) raising the temperature
b) increasing concentration of A
c) adding a catalyst
d) none of these
e) lowering the temperature
-- I picked C, although at first I thought it was B...
Thank you for helping, I really appreciate it :)
1. Yes, you have to do an ice table.
2. You picked d, but you thinkiit s either a or c?
3. You picked C, but you thougth it was B.
Years of experience tells my you are fishing for the right answers. And, the same years of experience tells me you have been doing this for years.
I will be happy to critique your thinking. Lay out why, and we can work through it, but I am not going to give it to you.
It's actually the end of my first year doing this topic hahah x)
for 1) i got 2.4 x 10^-9 for the Ka value. I understand what i over looked, thank you.
2) Well i just get very confused when especially with salts, acids, and bases. NH4Cl(aq) dissociates into NH4^+ and Cl^-. NH3/NH4^+ are conj acid/base pairs. and I know that NH3 is a weak base, making NH4^+ a strong acid, right? so the solution of this compound is acidic because.... (and this is where I got confused.
3) well I just need some reassurance here. It must be C, right? I'm pretty sure it is C. At first I thought it was B because in my notes it says change of concentrations of reactants does not change the K value of the reaction, but it's just the wording of the questions that is bugging me. is it the formula that they are asking about or the chemical reaction?
I think I got the reasoning for 2), it is acidic because it is the salt made from a strong acid and a weak base, therefore it is A (salt of weak base), not C (the salt is neutral itself, but slightly acidic in a solution). I believe this is the right answer. Am I wrong?
Sure! I'll try my best to explain and help you with these questions.
1) To determine the ionization constant, Ka, of the weak acid, we can use the pH of the solution and the formula for calculating the concentration of H3O+ ions from pH. The pH of a solution is given by the equation pH = -log[H3O+], where [H3O+] represents the concentration of H3O+ ions. By rearranging this equation, we can calculate [H3O+] as 10^(-pH).
In this case, the pH is given as 4.93. Therefore, [H3O+] = 10^(-4.93) = 7.02 x 10^(-5) M.
Now, we know that the acid is weak, so it does not completely ionize. Let's assume that the concentration of the weak acid is x M. Then, the concentration of the conjugate base (A-) will also be x M, as it is formed by the ionization of the acid. Therefore, at equilibrium, the concentration of [H3O+] will be approximately equal to x.
So, we can write the equation for Ka as:
Ka = ([H3O+][A-])/[HA] = (x)(x)/(0.057).
Now, substituting the value of [H3O+] = x = 7.02 x 10^(-5) M and [HA] = 0.057 M into the equation for Ka, we get:
Ka = (7.02 x 10^(-5))(7.02 x 10^(-5))/(0.057) = 9.67 x 10^(-7).
Comparing this value with the options given, we see that the closest answer is option (d) 2.8 x 10^(-7). Therefore, the correct answer is d) 2.8 x 10^(-7).
2) NH4Cl is the salt formed from the neutralization of a weak base (NH3) with a strong acid (HCl). Therefore, it can be classified as an acidic, basic, or neutral solution depending on the acid-base properties of its constituent ions.
In the case of NH4Cl, NH4+ is the conjugate acid of the weak base NH3, which means it can donate a proton (H+). Cl- is the conjugate base of the strong acid HCl, which does not have the ability to accept a proton (H+).
Since NH4+ can donate a proton in water, it will act as an acid, making the resulting solution acidic. Therefore, the correct answer is a) acidic because NH4Cl is the salt of a weak base.
3) The rate equation given is rate = k(A)(B)^2, where A and B represent the reactants in the reaction. The rate constant, k, depends on various factors such as temperature, concentration of reactants, and the presence of a catalyst.
In this case, raising the temperature, increasing the concentration of A, and adding a catalyst can all affect the rate constant, k. Therefore, options a) raising the temperature, b) increasing concentration of A, and c) adding a catalyst can all change k.
The factor that will NOT change the rate constant, k, is lowering the temperature. Lowering the temperature will slow down the reaction, but it will not affect the rate constant itself. So, the correct answer is e) lowering the temperature.
I hope these explanations have helped you! If you have any further questions, feel free to ask.