Arrange the following 0.10 M aq. sol. in order of increasing pH.
KOBr, HOBr, LiOH, HBrO4, NaBrO4
(Ka for HOBr = 2.5 x 10^-9, Ka for HBrO4 = 3.2 x 10^8)
I was wondering how to solve this. I know LiOH is a strong base so its pH is the highest. And I know that I can use ICE to individually solve for the 'x' then use that to get pH. But that's only the 2 whose Ka was given. Is there a way to solve this quickly?
KOBr, HOBr, LiOH, HBrO4, NaBrO4
Here's a crack at it with no calculations.
HBrO4 is the strong acid so pH = 1. This will be the lowest pH.
NaBrO4 is the salt of a strong base and strong acid; therefore, pH = 7
LiOH is a strong base so this is the highest @ pH = 13
HOBr is a weak acid so it should be between 1 and 7. Guess @ 4-6
KOBr will hydrolyze in H2O and will be basic so it will be between 7 and 13. Guess @ 10-12.
Check it out.
To quickly solve this problem, you can compare the relative acidities of the acids given based on their Ka values. The larger the Ka value, the stronger the acid, and the lower its pH. Here's how you can arrange the solutions in order of increasing pH:
1. LiOH: Since LiOH is a strong base, it would have the highest pH among the solutions. Therefore, it goes at the end of the list.
Order so far: [unknown]
2. HBrO4: HBrO4 has a Ka value of 3.2 x 10^8, which is significantly larger than the Ka value for HOBr. This indicates that HBrO4 is a stronger acid than HOBr. Therefore, HBrO4 goes before HOBr in the list.
Order so far: [HBrO4, unknown]
3. HOBr: The Ka value for HOBr is 2.5 x 10^-9, which is smaller than that of HBrO4. This means that HOBr is a weaker acid than HBrO4. Therefore, HOBr goes after HBrO4 in the list.
Order so far: [HBrO4, HOBr, unknown]
4. NaBrO4: NaBrO4 is a salt of a strong acid (HBrO4) and a strong base (NaOH). Salts of strong acids and strong bases are neutral, so they have a pH of 7. Therefore, NaBrO4 goes before HBrO4 in the list.
Order so far: [NaBrO4, HBrO4, HOBr, unknown]
5. KOBr: KOBr is also a salt of a strong acid (HBrO4) and a strong base (KOH). Like NaBrO4, it is also neutral and has a pH of 7. Therefore, KOBr goes before NaBrO4 in the list.
Order so far: [KOBr, NaBrO4, HBrO4, HOBr, unknown]
The final order of the solutions in increasing pH is:
KOBr, NaBrO4, HBrO4, HOBr, LiOH.
To quickly solve this problem, you can compare the strengths of acids and bases without calculating the individual concentrations of each species.
Step 1: Identify the strong acid/base
In this case, we know that LiOH is a strong base, so it will have the highest pH.
Step 2: Compare the acid strengths
For the remaining acids (KOBr, HOBr, HBrO4, NaBrO4), we can compare their acid strengths using their respective acid dissociation constants (Ka values).
- Ka(HOBr) = 2.5 x 10^-9
- Ka(HBrO4) = 3.2 x 10^8
The larger the Ka value, the stronger the acid. Therefore, HBrO4 is the stronger acid compared to HOBr.
Step 3: Arrange in increasing pH order
Now, we can arrange the remaining compounds in increasing pH order based on their acid strengths.
Lowest pH: HOBr
Followed by: NaBrO4
Followed by: KOBr
Highest pH: LiOH (strong base)
Therefore, the order of increasing pH for the given 0.10 M aqueous solutions is: HOBr, NaBrO4, KOBr, LiOH.