A 100.0 mL sample of a solution that is 0.100 M in HCl and 0.100 M in HCN is titrated with 0.100 M NaOH. Calculate the pH after the addition of the following volumes of NaOH: (b) 75.0 mL

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Determine the initial moles of HCl and HCN in the solution. Use the formula: \( \text{moles} = \text{concentration} \times \text{volume} \).

Calculate the moles of NaOH added using the same formula: \( \text{moles} = \text{concentration} \times \text{volume} \).

Identify the reaction that occurs. HCl, being a strong acid, will react completely with NaOH first: \( \text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O} \).

Determine the moles of HCl remaining after the reaction with NaOH. If NaOH is in excess, calculate the moles of NaOH remaining.

Since HCN is a weak acid, if NaOH is in excess, it will react with HCN: \( \text{HCN} + \text{NaOH} \rightarrow \text{NaCN} + \text{H}_2\text{O} \). Use the Henderson-Hasselbalch equation to find the pH: \( \text{pH} = \text{pK}_a + \log \left( \frac{[\text{A}^-]}{[\text{HA}]} \right) \), where \([\text{A}^-]\) is the concentration of the conjugate base and \([\text{HA}]\) is the concentration of the weak acid.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Acid-Base Neutralization

Acid-base neutralization is a chemical reaction where an acid reacts with a base to produce water and a salt. In this scenario, HCl (a strong acid) and HCN (a weak acid) are neutralized by NaOH (a strong base). Understanding the stoichiometry of this reaction is crucial for calculating the resulting pH after specific volumes of NaOH are added.

pH Calculation

pH is a measure of the hydrogen ion concentration in a solution, calculated using the formula pH = -log[H+]. In titration problems, the pH can change significantly depending on the amounts of acid and base present. After neutralization, the pH will depend on the remaining acid or the formation of the conjugate base, especially when weak acids are involved.

Buffer Solutions

Buffer solutions resist changes in pH upon the addition of small amounts of acid or base. In this case, the presence of HCN and its conjugate base (CN-) after partial neutralization creates a buffer system. Understanding how buffers work is essential for predicting the pH at various points during the titration, particularly after the addition of NaOH.

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Buffer Solutions

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