# Help in Chemistry!!

You are given an aqueous solution of hydrofluoric acid. (Recall that hydrofluoric acid is a weak acid; When you are given an aqueous solution of a weak acid you always assume that the dissociation reaction of the acid in water has reached equilibrium)

Predict whether the reaction will shift left ("left"), shift right("right"), or remain unchanged ("no change") upon the following disturbances.
Add solid sodium fluoride (assume no chnage in total volume)
Add solid sodium hydroxide (assume no chnage in total volume)
Add a drop of concetrated hydrochloric acid (assume no chnage in total volume)
Add solid iron(III) hydroxide (assume no chnage in total volume)
Add solid sodium nitrate (assume no chnage in total volume)
Add solid Barium fluoride (assume no chnage in total volume)

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1. HF + H2O ==> H3O^+ + F^-

This is a problem on LeChatelier's Principle. I like the way the books say it (the Principle); however, it's confusing to most students. Here is another way of saying it.
When a system at equilibrium is disturbed, the system will shift one way or the other, to UNDO what we've done to it. With that in mind, I will do the first few and leave the others for you.
Add solid NaF. So NaF is an ionic salt, it ionizes in soln as NaF ==> Na^+ +F^- which says we are increasing the F^-. So what will the reaction do? It will try to UNDO (decrease) the F^-. How can it do that? Remember it can shift to the right which gives MORE H3O^+ and MORE F^- OR it can shift to the left which will DECREASE H3O^+ and DECREASE F^-. Since adding F^- make more F^-, it will shift to the left since that's the only way to decrease it.

Add solid NaOH. That will increase OH^-, that will react with H3O^+, decreasing H3O^+ (to form H2O) so the reaction will shift to try to add H3O^+ back. How can it do that? By shifting to the right.

Adding a drop of concd HCl. That will increase H3O^+ so the reaction will shift to the left so as to UNDO (decrease H3O^+).

Some hints:
BaF2 is not very soluble.
NaNO3 is quite soluble.

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posted by paul
3. I'll bite. What about it? Are you concerned that I wrote the ionization with water? If so the solution said it was an aqueous soln; therefore, I just added the water in to show all of the ionization. That's what happens anyway. Or is it something else?

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4. so for BaF2 its going to shift to the left. The iron (3) nitrate is going to shift to the right. The sodium nitrate is not going to affect the reaction. And water will make it shift to the right. Is that right?

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posted by Kay
5. What is the density of water in units of mg/cm3
and mg/mL? (1=1000 mg)

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