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pH values for salts and other compounds

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Guest The X

Hi! I am going through one of my old paper 1s, and came across the question:

Which solution has the lowest pH value?
A. Aluminium sulfate
B. Sodium nitrate
C. Potassium chloride
D. Sodium ethanoate

The answer to the question is A. Could anyone explain why?

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Ya, its because of the size of the cation. If you look at the cations of those salts, Al is the most positive, 3+, therfore it will attract more hydroxide anions when in water. And the more OH- it attracts, the more H+ that are left in the water, so the acidity is extremely high, near 3. All you do when they ask these problems is look at the cations and look for the most positive one to see which one will create the most acidic solution and most negative for most basic solution.

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Guest The X
Aha, I see. I'm still a little curious though:

Aluminium sulfate has the formula Al[sub]2[/sub](SO[sub]4[/sub])[sub]3[/sub]. Broken up, we get two Al atoms, both with charge 3+. Does this mean they will attract three OH[sup]-[/sup] groups each?
And the (SO[sub]4[/sub]), which will have charge -1 each. Don't these attract the protons? I've always been a little unsure about just that.

Edit: Little interesting note though: It seems that we "non-VIPs" cannot send VIPs messages.

Edit 2: Off to bed now. :P Edited by The X

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Guest The X
Got another one here, this time it's bases.

Which compound of an element in period 3 reacts with water to form a solution with a pH greater
than 7?
A. SiO[sub]2[/sub]
B. SiCl[sub]4[/sub]
C. NaCl
D. Na[sub]2[/sub]O

The answer is D, Na[sub]2[/sub]O. I'm guessing that this time it's the anion charge which counts? Here it should be oxygen, with -2. But...?

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Oh sorry i didnt answer the one from before. Here let me try to explain:

Note the following:
1. Cations of the strong Arrhenius bases will not affect the pH.
2. SOLUBILITY RULES: for Ionic Compounds (Salts)
All OH- are insoluble except for IA metals, NH4+, Ca2+, Ba2+ , and Sr2+ (heavy IIA).
3. A cation that is the conjugate acid of a weak base will decrease the pH (behave as acids).
4. Other metal ions will cause a decrease in pH.

Therefore, since you have a base, a sulfate salt is an arrhanious base, then when paired with the other multiple choice answers, it would create a STRONG base, lowering the pH very little. But when paired with Aluminum, this one makes the base extremely weak, lowering the pH significantly. I have a great picture that we took in an experiment that i want you to see but you can't see uploads so i'll try to find one online similar and give you the url.

By the way, the reason none of the other answers are true is because 1) note that they all contain a strong anion (when i say strong remember that something is strong when it dissolves, so just look at solubility rules for strenght) 2) the rest of the answers other than aluminum are heavy first or second column cations, and aluminum is the only one that is all the way to the right.

As far as your new question goes, do yourself a favor, every time you see these type of questions, write the formula because its really easy and usually answers a lot of questions:

I'm not gonna write them all down, but to make things short, they all yield either NaOH or SiOH plus OH or HCL. Now, of course that the ones that yield HCL are not the answer because they'll just higher the pH, well lower it but you know what i mean :D . Now you have two answers left, NaOH + OH or SiOH + OH. This time you have to look at it the other way around, you want the equilibrium to stay on the left, so you WANT your conjugate base (NaOH) to be soluble. And comparing strenghts, NaOH dissolves completely becuase it's a IA metal BUT SiOH doesn't, making it a weak base. Now putting everything together:

1) you know the answer is not B or C beause those will yield HCL

so the only answers you have left are Na2O and SiO2, but one is a strong conjugate base and the other one is weak. If you pick the weak one, then it won't dissolve in water, making the solution slightly acidic, not above 7 like you need it to be. Now if you pick the strong one, NaOH, then you know it will dissociate completely and produce more hydroxide ions making the solution basic, above 7 on pH.

hope it helped :blink:

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Guest The X
Thanks a million! That helped a lot!
I'm going to do some thinking now, to completely understand it.

If you can't find an equivalent picture on the net, I guess you could upload it on a site like [url="http://imageshack.us/"]Imageschack[/url]

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