To determine which reaction yields a salt that has a pH less than 7, we need to look at the nature of the acids and bases involved in each reaction and how they combine to form salts.
1. KOH + HBr:
- KOH is a strong base.
- HBr is a strong acid.
- When a strong base reacts with a strong acid, the resulting salt solution is typically neutral (pH ~ 7).
2. HCOOH + KOH:
- HCOOH (formic acid) is a weak acid.
- KOH is a strong base.
- When a weak acid reacts with a strong base, the resulting salt solution is typically basic (pH > 7).
3. Ca(OH)_2 + H_2SO_4:
- Ca(OH)_2 is a strong base.
- H_2SO_4 is a strong acid.
- When a strong base reacts with a strong acid, the resulting salt solution is typically neutral (pH ~ 7).
4. NH_3 + HNO_3:
- NH_3 (ammonia) is a weak base.
- HNO_3 is a strong acid.
- When a weak base reacts with a strong acid, the resulting salt solution is typically slightly acidic (pH < 7).
Based on this analysis, the reaction that yields a salt with a pH less than 7 is NH_3 + HNO_3. The resulting ammonium nitrate (NH_4NO_3) forms a slightly acidic solution.
Thus, the answer is:
NH_3 + HNO_3.
This corresponds to the reaction index 4 in the given options.
