# chem

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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
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 :)

• chem -

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.

• chem -

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?

• chem -

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?