To determine which reaction yields a salt that has a pH < 7, we need to consider the type of acid and base involved in each reaction. The pH of the resulting solution depends on whether the salt formed is neutral, acidic, or basic.
Let's examine each reaction step-by-step:
1. KOH + HBr:
- KOH is a strong base.
- HBr is a strong acid.
- The reaction forms KBr (potassium bromide) and water.
- Strong base + strong acid usually produces a neutral salt, which means the resulting solution has a pH of around 7.
2. HCOOH + KOH:
- HCOOH (formic acid) is a weak acid.
- KOH is a strong base.
- The reaction forms HCOOK (potassium formate) and water.
- Weak acid + strong base usually produces a basic salt, meaning the resulting solution has a pH > 7.
3. Ca(OH)_2 + H_2SO_4:
- Ca(OH)_2 (calcium hydroxide) is a strong base.
- H_2SO_4 (sulfuric acid) is a strong acid.
- The reaction forms CaSO_4 (calcium sulfate) and water.
- Strong base + strong acid usually produces a neutral salt, so the resulting solution has a pH of around 7.
4. NH_3 + HNO_3:
- NH_3 (ammonia) is a weak base.
- HNO_3 (nitric acid) is a strong acid.
- The reaction forms NH_4NO_3 (ammonium nitrate).
- Weak base + strong acid usually produces an acidic salt, which means the resulting solution has a pH < 7.
Based on this analysis, the reaction that yields a salt with a pH < 7 is:
NH_3 + HNO_3.
Therefore, the correct reaction is:
4. NH_3 + HNO_3.
