Workshop 5: Titration Curves A pH or titration curve can be produced by plotting

Question

Workshop 5: Titration Curves A pH or titration curve can be produced by plotting the pH of a solution (containing an analyte) versus the volume of added The equivalence or end point of a titration occurs when enough titrant has been added to consume all the acid or base initially present in the analyte.You know you have reached the endpoint when you see a significant change in the pH of the solution. Part Lets begin by analyzing a titration curve (2 pts) When your analyte is a weak acid or base the titration curve shows the effects of buffering before the equivalence point. The midpoint of the titration occurs in the buffer zone and is the point at which the pH is equal to the pKa of the analyte. In the case of a weak acid-strong base titration the pH at the equivalence point will always be higher than 7. This is because of the basic properties of the anion (the conjugate base of the weak acid) that is present in the solution at this point. To the right is a titration curve for the Titration of unknown acid titration of 25.0 mL of unknown compound X with 0.200 M NaOH. Use this 14 titration curve to answer the following 12 questions. (a) What is the identity of compound X? pH HINT: Use the Chempendix on Sapling to look at the Ka values for possible weak acids. (1 pt) 6.309 x lo 10 20 30 40 50 un ouc oud ,accordin volume of 0.200 M NaoH added, mL to the Chempendix table of Ra of is the owy aded with a ka valuu 6.3 x so cow pound X ts ben 201ca cud Naort M xV (b) What is o Mones of the starting concentration of compound X? (1 pt) This is l: l mone raho 0.0002 m 0.24 M 0,025 L Before you move on to part 2 please make sure EVERYONE in your group is comfortable predicting the identity and starting concentration of an analyte from the midpoint of a titration curve. This is a very important skill! You will need the correct Benzoic acid concentration to complete part 2 of the workshop What can you do to make sure you have the correct answer?

Explanation / Answer

V = 0

At V = 0, there is plenty of molecular benzoic acid, and some H+ due to dissociation of acid, the pH active is only H+ ions and pH as stated in the graph is 2

V = 20

At V = 0, there is plenty of molecular benzoic acid, and also H+ due to dissociation of acid, the pH active is only H+ ions and A- due to teh buffer formaiton

pH shown in graphis about 4.2

V = 30.75

this is equivalence point, there is mainly Benzoate ions and OH- due to the hydorlysis

the pH active is actually benzoate, since it will release OH- ions in solution

pH in the grah ios about 8.5

V = 40

major species are mainly OH due to excess addition

so pH active is OH-

pH = 12.56 approx

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