Kaplan General Chem Subject Test 5

[Meas]

Kaplan General Chem Subject Test 5:

As mentioned in previous threads, I am posting questions I have about subject tests that I did. Hope to get responses you all!

14) Which of the following pairs of elements would form the most ionic compound?

Answer choices:
Au & S
Au & Cl
Pt & Cl
La & Br
La & Cl

A: La & Cl

--Should we look at the oxidization states & choose the pair with the greater difference in oxidation states? If so, for the last two answer choices, both Br & Cl have ox. state of –1.

15) AgOH is a slightly soluble ionic compound whose Ksp = 1.5 x 10^-8 at 20C. What is the minimum pH at which AgOH will precipitate from a solution, 0.010M in Ag2+?

Answer: 8+log(1.5)

--You know how generally Ksp for bases are: A- + H2O -> HA + OH-. For AgOH, I am not sure how to write an equation like this. Would it be OH- + H2O -> H2O + OH-? If so, Ag2+ will only be a spectator ion & is not included in the Ksp equation.

Thanks in advance!

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[dentrilla]

Kaplan General Chem Subject Test 5:

As mentioned in previous threads, I am posting questions I have about subject tests that I did. Hope to get responses you all!

14) Which of the following pairs of elements would form the most ionic compound?

Answer choices:
Au & S
Au & Cl
Pt & Cl
La & Br
La & Cl

A: La & Cl

--Should we look at the oxidization states & choose the pair with the greater difference in oxidation states? If so, for the last two answer choices, both Br & Cl have ox. state of –1.

15) AgOH is a slightly soluble ionic compound whose Ksp = 1.5 x 10^-8 at 20C. What is the minimum pH at which AgOH will precipitate from a solution, 0.010M in Ag2+?

Answer: 8+log(1.5)

--You know how generally Ksp for bases are: A- + H2O -> HA + OH-. For AgOH, I am not sure how to write an equation like this. Would it be OH- + H2O -> H2O + OH-? If so, Ag2+ will only be a spectator ion & is not included in the Ksp equation.

Thanks in advance!

Not 100% on the first one but I think ionic bond strength has to do with differences in electronegativity. The biggest difference will yield the strongest ionic bond. (this works for this question, La and Cl are farthest away on periodic table) Please correct me if I'm wrong.


For the precipitate question, remember that when your Ksp is equal to your Qsp, precipitate starts to form. When Qsp > Ksp, reaction is pushed towards the solid and when Qsp < Ksp reaction is pushed towards the ions (no precipitate formed)

For this question, find out the [OH-] by calculating

Ksp = [Ag+][OH-]
1.5x10^-8 = (0.01M)(x)
x=[OH-]=1.5x10-6

therefore, your pOH is just under 6, meaning your pH must be just over 8.
(no calculater needed!)

 

[Meas]

Hey dentrilla,

Thank you so much for your reply! I posted 5 threads on kaplan general chem practice tests (1 - 5) & you are the only who replied so far! From the high # of views shown, it seems that people either have similar questions as I do OR they have other questions about the practice test. I went through all the general chem questions & the ones that I posted are the only ones that I am really stuck on. So, feel free to ask me questions that I did not list. For those of you who do know the answers to the ones I posted, please give me a hand!

For that first question, I checked the eletronegativity table & the pair (La & Cl) do have the greatest difference. Now that I think about this, this makes sense b/c the more eletronegative an atom (thus, the greater difference), the more electrons it can pull in & thus making a positive & a negative end. Ionic bonding is when we have one cation + one anion. So, you are right about electronegativity.

I am not sure if ionic bonding is only between metals & nonmetals though.

 

[OC1314]

Ionic bonds bt metals and nonmetals