CHEM 210 ACTUAL EXAM WITH

CHEM 210 ACTUAL EXAM WITH

200 QUESTIONS AND CORRECT

ANSWERS GRADED A (BEST FOR

MIDTERM / FINAL EXAM PREP)

Define the end point of a titration. - ANSWER-The equivalence point of a titration is reached when the moles of titrant added are stoichiometrically equal to the moles of analyte present in solution. This is observed on the titration curve as the point in the center of the steepest part of the curve, which correlates to the greatest concentration (pH) change per volume of added titrant for the titration.The equivalence point of a titration is the theoretical result of a titration, but what is actually measured is the end point. The end point is when a sudden change occurs in a physical property of the analyte solution that implies equivalence or thereabouts. A common example is the color change of an indicator.

In the Fajans titration of Hg2+2Hg22+, NaClNaCl is added to produce the precipitate Hg2Cl2Hg2Cl2. The end point of the titration is detected with bromophenol blue.What charge do you expect the precipitate to have after the equivalence point? - ANSWER-Negative

In the Fajans titration of Hg2+2 with NaCl, the solution contains excess Hg2+2 before the equivalence point. The excess Hg2+2 adsorbs onto the surface of the precipitate, imparting a positive charge onto the precipitate. After the equivalence point, there is excess Cl−Cl− in solution, which adsorbs to the surface of the precipitate, imparting a negative charge onto the precipitate.

Describe how the Volhard method can be used to determine the CN−CN− concentration of a solution by placing the steps in the order they occur. - 1 / 4

ANSWER-In the Volhard method, a known excess of standard AgNO3AgNO3 is added to the CN−CN− in 0.5 M HNO3HNO3 solution while stirring vigorously, resulting in the precipitation of AgCNAgCN.

Ag+(aq)+CN−(aq)⟶AgCN(s)Ag+⁢(aq)+CN−⁢(aq)⟶AgCN(s)

Vigorous stirring is required to prevent excess Ag+Ag+ from becoming trapped in the precipitate as it forms. The next step is to filter off the AgCNAgCN precipitate and wash it with dilute HNO3HNO3. This step is required because AgCNAgCN is more soluble than AgSCNAgSCN, which is formed later in the titration. If AgCNAgCN is not removed from the solution, the end point will slowly fade as AgCNAgCN redissolves and is replaced by AgSCNAgSCN. After the precipitate is removed, Fe(NO3)3Fe(NO3)3 solution is added to the filtrate to give a concentration of 0.2 M Fe3+Fe3+. The excess Ag+Ag+ in the filtrate is then titrated with a standard KSCNKSCN solution.

Ag+(aq)+SCN−(aq)⟶AgSCN(s)Ag+⁢(aq)+SCN−⁢(aq)⟶AgSCN(s)

Once all of the Ag+Ag+ has been consumed, SCN−SCN− reacts with Fe3+Fe3+ to form a red complex, which signifies the end point of the back titration.

Fe3+(aq)+SCN−(aq)⟶FeSCN2+(aq)Fe3+⁢(aq)+SCN−⁢(aq)⟶FeSCN2+⁢(aq)

The amount of SCN−SCN− required for the back titration tells you the excess amount of Ag+Ag+ from the reaction with CN−CN−. Because the total amount of Ag+Ag+ added to the solution is known, the amount that reacted with CN−CN− can be determined.

The Fajans titration uses an adsorption indicator to detect the endpoint of a titration in which a precipitate is formed. For the titration of Cl−Cl− by Ag+Ag+ , insoluble 2 / 4

AgClAgCl is formed. Indicate what is happening at each step of the titration by completing each statement.

During the titration ...

• a Cl−Cl− solution is added to an Ag+Ag+ solution.
• an Ag+Ag+ solution is added to a Cl−Cl− solution.

At the endpoint, just past the equivalence point,

• an excess of Ag+ ions imparts a positive charge to the surface of the precipitate.
• an excess of Cl−Cl− ions imparts a positive charge to the surface of the

precipitate

• an excess of Cl−Cl− ions imparts a negative charge to the surface of the

precipitate.

• an excess of Ag+Ag+ ions imparts a negative charge to the surface of the

precipitate.

When the endpoint is detected by a dye color change,

• a cationic dye binds to the positive surface o - ANSWER-The Fajans titration

uses an adsorption indicator to detect the endpoint of a titration in which a precipitate is formed. For the titration of a Cl−Cl− solution by Ag+Ag+ , insoluble AgClAgCl is formed. Just before the equivalence point, there is still an excess amount of Cl−Cl− in solution. After the equivalence point, there is an excess of Ag+Ag+ ions in solution, which imparts a positive charge to the surface of the precipitate. The endpoint of the titration is detcted using an anionic dye, which bonds to the positively charged surface of the precipitate.

Sample pretreatment plays an important role in gravimetric analysis because the analyte must meet certain requirements to be measured gravimetrically.What characteristics must an analyte have to be measured using traditional gravimetric analysis? 3 / 4

able to be precipitated.soluble.insoluble.in basic solution.in a mineral-based sample.in acidic solution. - ANSWER-For an analyte to be measured gravimetrically, it must be in a form that is both soluble and can be precipitated upon addition of a precipitant. An insoluble analyte would not form a precipitate upon reaction with a precipitant, so the analyte needs to initially be soluble. The acidity of the solution during gravimetric analysis is only important in how it affects the precipitation of the analyte and the precipitant. The analyte can come from any sample source as long as it is soluble and can be precipitated.

Which of the properties are desirable of a gravimetric analysis precipitate?

be of known composition insoluble easily filterable small particle size forms a colloidal suspension - ANSWER-Ideally a gravimetric analysis precipitate is insoluble, pure, easily filterable, and of known composition. A precipitate that is easily filterable and insoluble is easier to isolate from the solution. Larger particles are preferred over small ones because small particles can clog or pass through the filter. Also, large particles have less surface areas on which impurities can attach.A precipitate that is pure and of known composition can be used to quantify the amount of analyte in the original solution.

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