How can we determine the equivalence point ( or end point ) in the curves in titration ( strong acid/weak base and weak acid/strong base )
Do you mean how to calculate the pH at the equivalence point or how they look? Here is how they look; i.e., its' the mid-point of the almost vertical part of the curve.
https://www.google.com/search?q=titration+curves+images&client=firefox-a&hs=PS0&rls=org.mozilla:en-US:official&channel=sb&tbm=isch&tbo=u&source=univ&sa=X&ei=CIMSVPy7I4S1sQSFvYHACQ&ved=0CB8QsAQ&biw=1006&bih=602&dpr=1.25
If how you calculate it you "hydrolyze" the acid or base salt; e.g., for weak acid/strong base such as HAc where HAc is acetic acid, you have
HAc + NaOH --> NaAc + H2O. So at the equivalence point you have ONLY NaAc and H2O so the pH is determined by the hydrolysis of the salt, in this case it is
.........Ac^- + HOH ==> HAc + OH^-
I........C...............0......0
C........-x..............x......x
E........C-x............x.......x
Then Kb for Ac^- = (Kw/Ka for HAc) = (x)(x)/(C-x). You will know the concn of Ac^- and Kw and Ka for HAc and you calculate x = OH^- and convert to pH. That gives you the pH at the equivalence point.
To determine the equivalence point or end point in titration curves for strong acid/weak base and weak acid/strong base reactions, you can follow these steps:
1. Understand the concept of equivalence point: The equivalence point is the point in a titration where the moles of the acid being titrated are equal to the moles of the base that has been added. At this point, the solution is neutral, and further addition of the base will result in excess base.
2. Observe the titration curve: Plot a graph of pH (or any relevant variable) against the volume of the titrant added. During titration, the pH of the solution changes as the titrant is added. Typically, the pH starts at a low value, increases gradually, and then sharply increases or reaches a plateau near the equivalence point.
3. Identify the steep slope or sharp increase: Look for the point on the titration curve where there is a noticeable change in the gradient or slope. This steep slope or sharp increase is an indication of reaching the equivalence point.
4. Use an indicator (optional): In some cases, an indicator can be used to help identify the equivalence point. An indicator is a substance that changes color at a certain pH range. It is chosen so that its color change occurs around the expected pH of the equivalence point. The color change indicates that the solution is near or at the equivalence point.
5. Perform calculations (optional): If a precise determination of the equivalence point is required, you can use calculations based on stoichiometry. This involves determining the number of moles of the acid or base from the volume and concentration and using the balanced chemical equation to find the stoichiometric ratio. The equivalence point is reached when the calculated moles of acid and base are equal.
Note: The process may differ slightly for different titration methods or specific experimental setups. Always refer to the specific instructions provided by your instructor or follow established laboratory procedures.