[adc626]

I have to rank these compounds in terms of increasing pH:

(a) HI, HF, NaF, NaI
(b) NH4Br, HBr, KBr, NH3
(c) C6H5NH3NO3, NaNO3, NaOH, HOC6H5, KOC6H5, C6H5NH2, HNO3

My first try on (a) was NaF, NaI, HF, HI, but apparently I'm wrong? I haven't the slightest clue as to how to tell the pH of compounds.


Also, I would really appreciate if someone could give me a hint as to how to tackle this problem:

A solution is prepared by adding 54.0 mL of 0.050 M HCl to 156.0 mL of 0.30 M HNO3. Calculate the concentrations of all species in this solution.

[H+]
[OH-]
[Cl-]
[NO3-]

[constant thinker]

You could look up pK_a values to determine pH. The lower the pK_a the lower the pH (more acidic). This works for the things that are actually acids. You have compounds that are salts mixed in, though.

Alternatively you can break it down by what you have. Salts (like NaF and NaI in a) are always going to have a higher pH than their acid counter parts (HF and HI respectively). When it comes to determining which salt will have a higher or lower pH, I'm sorry I can't help you there. I forget how to tell. To tell which acid will have the lowest pH, you can use a general rule of thumb for simple acids that are just HX, X being just one atom that's sufficient enough to cause polarity. The greater the difference in electronegativity between H and whatever X is the stronger the acid (lower the pH).

In (b) you have a combination of acid, base, and two salts. Try to identify which two are the salts, which on is the acid, and which one is the base. Post what you think.

(c) is more complicated so start with (a) and (b).
-----
For you other problem you have to make an assumption (and I'm assuming your knowledge of chem is relatively limited).

You can assume that since HCl and HNO₃ are both strong acids,they will completely dissociate.

If I were you, I'd start by calculating the number of moles of HCl, and HNO₃ in solution.
Start with that, then try going as far as you can.


Hope I could help you some.

[enahs]

The strength of a Bronsted acid depends on the extent that a proton can be removed from it and transfered to a base.

As we go down a column in the periodic table, the acidity increases. This is mainly due to the decrease in effectiveness of the bond strength. The overlap of the Hydrogen 1s orbital and the orbitals of successively larger and larger elements in the column becomes less effective. The less effective the overlap, obviously the weaker the bond, and thus the stronger the acids.


As we go from left to right in a period on the periodic table, the acidity increases. There is some bond strength differences, but the man cause is the increasing in electro-negativities. The electronegativity effects the polarity of the bond as well as the relative stability of the anion (conjugate base) that forms with the loss of a proton.

Now, do not forget about the opposite direction, the less acidic it is the more basic it is. The stronger the acid the weaker the conjugate base and the weaker the acid the stronger the conjugate base, and reverses, etc.



Examples:
pKa of Halogens
H-F 3.2
H-Cl -7
H-Br -9
H-I -10
As we go down the column, the acidity increases. And it works for other columns.
H2O 15.7
H2S 7.0
H2Se 3.9


Across a period.
H3C-H 48
H2N-H 38
HO-H 15.7
F-H 3.2


In short, acidity increases on periodic table
--->
    |
    |
    \/


Now, take that information, and what should be your general knowledge on periodic trends of electro negativities, bond sizes and strength's, etc, and answer the questions.



Also note, if you are have a hard time deciding about which two acids are stronger, then look at their conjugate bases if it is easier. The stronger the conjugate base the weaker the acid and vice-versa. Some questions are easier to approach that way.

[adc626]

Thank you so much for your help.

on (b), I'm guessing HBr is the acid, NH3 the base, and the other two are salts? Will salts always have a higher pH?

Heh, my knowledge of chemistry really shouldn't be that limited, but I have a lot of catching up to do in class. Again, thanks.

Also, thank you enahs, for the general rules in determining acidity! This will prove most helpful come test time.

[AWK]

on (b), I'm guessing HBr is the acid, NH3 the base, and the other two are salts? Will salts always have a higher pH?

Salts may be neutral, or slightly acidic, or slightly basic (slightly here means less than stronger acid or base from which hydrolyzing salt was obtained)