Multiple Choice
A 10.0 mL sample of 0.200 M hydrocyanic acid (HCN) is titrated with 0.0998 M NaOH. What is the pH at the equivalence point? For hydrocyanic acid, pKₐ = 9.31.
18. Aqueous Equilibrium
Titrations: Weak Acid-Strong Base
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A 30.0-mL volume of 0.50 M CH3COOH (Ka=1.8×10⁻⁵) was titrated with 0.50 M NaOH. Calculate the pH after the addition of 30.0 mL of NaOH at 25 °C.
A
pH = 3.75
B
pH = 8.72
C
pH = 4.76
D
pH = 7.00
Verified step by step guidance1
Start by identifying the type of titration: This is a titration of a weak acid (CH3COOH) with a strong base (NaOH). The equivalence point occurs when the moles of acid equal the moles of base added.
Calculate the initial moles of CH3COOH using the formula: \( \text{moles} = \text{concentration} \times \text{volume} \). For CH3COOH, \( \text{moles} = 0.50 \text{ M} \times 30.0 \text{ mL} \). Convert mL to L by dividing by 1000.
Calculate the moles of NaOH added using the same formula: \( \text{moles} = 0.50 \text{ M} \times 30.0 \text{ mL} \). Again, convert mL to L by dividing by 1000.
Since the moles of CH3COOH equal the moles of NaOH added, the solution is at the equivalence point. At this point, the solution contains only water and the salt (CH3COONa), and the pH is determined by the neutralization reaction.
At the equivalence point of a weak acid and strong base titration, the pH is typically around 7.00 because the salt formed does not affect the pH significantly, and the solution is effectively neutral.
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