Frequently Asked Question

This problem involves the reaction of a weak acid (HCN) with a weak base (NH3). Here's how to approach it:
1. Identify the Reaction:
The reaction between HCN and NH3 forms the salt ammonium cyanide (NH4CN) and water:
HCN(aq) + NH3(aq) NH4+(aq) + CN-(aq)
2. Determine the Limiting Reactant:

Calculate the moles of each reactant:

Moles of HCN = (20 mL)
(0.25 mol/L)
(1 L/1000 mL) = 0.005 mol

Moles of NH3 = (50 mL)
(0.1 mol/L)
(1 L/1000 mL) = 0.005 mol

Since the moles of HCN and NH3 are equal, neither is in excess. They will completely react to form the salt.
3. Calculate the Concentration of the Salt:

The total volume of the solution is 20 mL + 50 mL = 70 mL
The concentration of NH4CN is:

[NH4CN] = (0.005 mol) / (70 mL)
(1000 mL/L) = 0.0714 M
4. Consider the Hydrolysis of the Salt:

NH4CN is a salt formed from a weak acid (HCN) and a weak base (NH3). It will undergo hydrolysis, meaning the ions will react with water.
NH4+ will react with water to form H3O+ (acidic):
NH4+(aq) + H2O(l) H3O+(aq) + NH3(aq)
CN- will react with water to form OH- (basic):
CN-(aq) + H2O(l) HCN(aq) + OH-(aq)
5. Determine the pH:

To determine the pH, we need to consider the equilibrium constants for the hydrolysis reactions of NH4+ and CN- (Ka for HCN and Kb for NH3). This requires the use of the equilibrium constant expression and an ICE table to solve for the concentrations of H3O+ and OH-. Then, we can calculate the pH using the appropriate equation (pH = -log[H3O+] or pOH = -log[OH-] and then pH = 14 - pOH).
a. Degree of the Resulting Salt Formed:

Since all the reactants react to form the salt, the degree of the resulting salt formed is 100%.
b. pH of the Resulting Solution:

The pH of the resulting solution will be determined by the relative strengths of the acid and base involved in the hydrolysis of the salt. Since HCN and NH3 have similar Ka and Kb values, the pH will be close to neutral (around 7). However, it will be slightly acidic due to the slightly stronger acidic nature of HCN compared to the basic nature of NH3. To determine the exact pH, you'd need to perform the calculations mentioned in step 5.
Note: This explanation provides the general steps and concepts. The actual calculation for the pH requires knowledge of the Ka of HCN and the Kb of NH3.