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Rank these compounds in terms of increasing pH value

Submitted by minidee on Fri, 05/16/2008 - 22:47

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

thanks smart people!


Rank first by acids -  salt  -  base

Within the acid and base categories rank strong and weak  (strong acids and bases are completely ionized in solution, weak acids and bases are partially ionized).  The strength of weak acids and weak bases are indicated by a table of Ka and Kb values.  If an acid or base is listed with a Ka or Kb value it is weak.  The smaller the value the weaker the substance.

This results in:    strong acid  -  weak acid  -  salts  -  weak bases  -  strong bases

Finally, rank salts according to whether they are acidic salts, neutral salts, or basic salts.  This can be determined by looking at what acid and base went into forming the salt in a neutralization reaction (for instance, NaCl is formed from hydrochloric acid (HCl)  and sodium hydroxide (NaOH).  If the acid and base that formed the salt are both strong, the salt will be neutral.  If the acid is strong and the base is weak, the salt will be acidic.  If the acid is weak and the base is strong the salt will be basic.  If both the acid and base are weak, you need to consult the Ka and Kb values to decide which is stronger and apply the rules above, although the salt will be very close to neutral.

So, your final order of ranking should be:

strong acid - weak acid - acidic salt - neutral salt - basic salt - weak base - strong base

Applying that to (a): 

HI is a strong acid  -  HF is a weak acid  - NaI is a neutral salt (formed from a strong acid HI and a strong base NaOH) -                    NaF is a basic salt (formed from a weak acid HF and a strong base NaOH)
                           


I'm still having trouble with part c simply because I'm not familiar with some of the species. can you help me?


(c) C6H5NH3NO3, NaNO3, NaOH, HOC6H5, KOC6H5, C6H5NH2, HNO3

C6H4NH3NO3 is a salt of C6H4NH3OH (analogous to NH4OH where one of the H atoms has been replaced by a benzene ring) a weak base and nitric acid, a strong acid.

NaNO3 is the salt formed from NaOH and HNO3 (strong acids and bases)

NaOH is a strong base

HOC6H5 is phenol  (aka carboxalic acid) and is weakly acidic

KOC6H5 is the salt formed when phenol (weak acid) is neutralized with KOH (strong base)

C6H5NH2 is analogous to NH3 (one H has been replaced by a benzene ring).  It should behave like NH3 and be a weak base.

HNO3 is nitric acid and is a strong acid.