To determine which reaction yields a salt that has a pH less than 7, we need to understand the nature of the reactants and products involved in each reaction.
1. KOH + HBr → KBr + H2O:
- KOH is a strong base.
- HBr is a strong acid.
- The reaction of a strong base with a strong acid generally produces a neutral salt. KBr would be neutral with a pH = 7.
2. HCOOH + KOH → HCOOK + H2O:
- HCOOH (formic acid) is a weak acid.
- KOH is a strong base.
- The reaction of a weak acid with a strong base generally produces a basic salt. HCOOK would make the solution slightly basic with pH > 7.
3. Ca(OH)2 + H2SO4 → CaSO4 + 2 H2O:
- Ca(OH)2 is a strong base.
- H2SO4 is a strong acid.
- Similar to the first reaction, strong base reacting with strong acid usually produces a neutral salt. CaSO4 would be neutral with pH = 7.
4. NH3 + HNO3 → NH4NO3:
- NH3 (ammonia) is a weak base.
- HNO3 is a strong acid.
- The reaction of a weak base with a strong acid generally produces an acidic salt. NH4NO3 would make the solution acidic with pH < 7.
Given these reactions and their products:
- The salt that forms and has a pH < 7 is NH4NO3 from the reaction of NH3 (weak base) with HNO3 (strong acid).
Therefore, the reaction that yields a salt with a pH < 7 is:
[tex]\[ \boxed{NH_3 + HNO_3 \rightarrow NH_4NO_3} \][/tex]
Hence, the answer is the fourth reaction.
