Question

the table below lists some acids and bases

strong acids | weak acids | weak bases | strong bases
hbr | ch₃cooh | nh₃ | naoh
hno₃ | hcooh | nh₄oh | koh
h₂so₄ | | | ca(oh)₂

which reaction yields a salt that has a ph < 7?

○ koh + hbr
○ hcooh + koh
○ ca(oh)₂ + h₂so₄
○ nh₃ + hno₃

Explanation:

Step1: Recall salt pH rules

• Strong acid + strong base → neutral salt (pH = 7)
• Strong acid + weak base → acidic salt (pH < 7)
• Weak acid + strong base → basic salt (pH > 7)
• Weak acid + weak base → depends on \( K_a \) and \( K_b \)

Step2: Analyze each option

• Option 1: \( KOH \) (strong base) + \( HBr \) (strong acid) → neutral salt (pH = 7)
• Option 2: \( HCOOH \) (weak acid) + \( KOH \) (strong base) → basic salt (pH > 7)
• Option 3: \( Ca(OH)_2 \) (strong base) + \( H_2SO_4 \) (strong acid) → neutral salt (pH = 7)
• Option 4: \( NH_3 \) (weak base) + \( HNO_3 \) (strong acid) → acidic salt (pH < 7)

Answer:

\( NH_3 + HNO_3 \)