Which of the following 0.5 M aqueous salt solutions will have a pH of 7.0 at LiF RbBr NaClO4 NH4Cl A) LiF only B) NaClO4 only C) LiF and RbBr D) RbBr and NaClO4 <———— E) NH4Cl only Answer: D My question: I understand that LiF is a strong base + weak acid, so it is not neutral. I also understand that NH4Cl is a weak base and strong acid, so it is not neutral. I understand that NaClO4 is a strong base and a strong acid, so it is neutral. But for RbBr, Br is a strong acid, so why is Rb considered, I guess in this case, to be a strong base? I thought 1A elements were NEUTRAL? Can someone please explain to me the conditions when 1a/1b ions are neutral and when they are considered basic? Is it (aq) vs salt? Ahhhhhhhhhhhhhhh help, thanks
The Correct Answer and Explanation is:
You’re asking an excellent and nuanced question about salt solutions and their pH behavior in water. Let’s break it down clearly:
The key to predicting whether a salt solution is acidic, basic, or neutral lies in understanding how the ions from the salt interact with water (hydrolyze).
Group 1A and 2A Metals:
Cations from Group 1A (alkali metals) and Group 2A (alkaline earth metals) like Li⁺, Na⁺, K⁺, Rb⁺, Ca²⁺, Sr²⁺, etc., do not hydrolyze in water. They are spectator ions. They come from strong bases like NaOH or RbOH, which fully dissociate and do not react with water, so they do not affect pH. That means:
- Rb⁺ is neutral, not basic.
- Same goes for Li⁺, Na⁺, etc.
Anion Consideration:
Now check the anion of the salt.
- Br⁻ comes from HBr, a strong acid. Strong acid conjugate bases like Br⁻ do not hydrolyze, so they are also neutral.
- ClO₄⁻ comes from HClO₄, another strong acid — also neutral.
So, if both the cation and anion of a salt come from strong acid and strong base, the resulting solution is neutral (pH = 7).
Now, Your Specific Case:
- LiF → Li⁺ (neutral) and F⁻ (from HF, a weak acid) → F⁻ hydrolyzes, so basic solution.
- RbBr → Rb⁺ (neutral) and Br⁻ (neutral) → neutral solution.
- NaClO₄ → Na⁺ (neutral) and ClO₄⁻ (neutral) → neutral solution.
- NH₄Cl → NH₄⁺ (from a weak base, NH₃) and Cl⁻ (neutral) → NH₄⁺ hydrolyzes → acidic solution.
Conclusion:
The correct answer is D) RbBr and NaClO₄, because both their ions are neutral, and thus their 0.5 M aqueous solutions will have pH = 7.0.
Your confusion came from thinking 1A metals can be basic — but they are actually neutral in water. The only thing that makes a salt solution acidic or basic is whether either the cation or anion hydrolyzes, not whether the salt is aqueous or solid.
