2.5 ml of 0.4 M weak mono acidic base (Kb=10^-12) is titrated with 2/15M HCl in water. Find conc. of H+ at equivalence point. (assume T=25 degree C).... here after finding the volume required of hcl which is 7.5ml ...how do i find the conc of salt?...in my book its written [salt]=c=0.4x2.5/10..shouldn't this be conc. of the base???...plz explain how does this give conc of salt??

To tackle your question about the titration of a weak monoacidic base with hydrochloric acid (HCl), let's break it down step by step. You’ve already determined that you need 7.5 mL of HCl to reach the equivalence point, which is a great start. Now, let’s clarify how to find the concentration of the salt formed during this reaction.

The Reaction Overview

When you titrate a weak base with a strong acid, the weak base reacts with the acid to form a salt and water. In your case, the weak base (let's call it B) reacts with HCl to form the salt (BHCl) and water:

• B + HCl → BHCl + H2O

Understanding the Concentration of Salt

At the equivalence point, all of the weak base has reacted with the strong acid. The concentration of the salt formed can be calculated based on the stoichiometry of the reaction and the initial concentrations of the reactants.

Calculating Moles of the Weak Base

First, let’s find the number of moles of the weak base you started with:

• Volume of weak base = 2.5 mL = 0.0025 L
• Concentration of weak base (C) = 0.4 M
• Moles of weak base = Concentration × Volume = 0.4 mol/L × 0.0025 L = 0.001 mol

Finding the Moles of Salt Formed

Since the reaction between the weak base and HCl is a 1:1 reaction, the moles of salt formed (BHCl) will be equal to the moles of the weak base that reacted:

• Moles of salt (BHCl) = Moles of weak base = 0.001 mol

Calculating the Concentration of the Salt

Now, to find the concentration of the salt at the equivalence point, we need to consider the total volume of the solution after the titration:

• Volume of HCl used = 7.5 mL = 0.0075 L
• Total volume at equivalence point = Volume of weak base + Volume of HCl = 2.5 mL + 7.5 mL = 10 mL = 0.01 L

Now, we can calculate the concentration of the salt:

• Concentration of salt (C_salt) = Moles of salt / Total volume = 0.001 mol / 0.01 L = 0.1 M

Clarifying the Confusion

It seems there was a misunderstanding in your book regarding the concentration of the salt. The expression [salt] = C × V / 10 likely refers to the moles of the weak base divided by the total volume after the reaction, which gives you the concentration of the salt formed, not the concentration of the base itself. The factor of 10 in your book might be a simplification or a specific context that needs clarification.

In summary, at the equivalence point, the concentration of the salt formed from the reaction of the weak base with HCl is indeed 0.1 M, derived from the moles of the weak base and the total volume of the solution. This approach ensures that you accurately account for the stoichiometry of the reaction and the dilution effect of adding the titrant.