Question

Purpose: To calculate the amount of AgNO₃ present in an unknown solution of water through the process of gravimetric analysis with NaCl. 

 

Background:

Have you ever had difficulty lathering soap or find that the scum in your shower constantly needs to be removed? These are signs of “hard water.” Soap does not lather well in hard water because metal ions, such as Ca²⁺, form precipitates, creating “soap scum.” A precipitate is an insoluble compound that forms when soluble ions in separate solutions are mixed together. Because this happens, soap is a less effective cleanser in hard water. Even laundry can appear dingy or feel rough when washed in hard water.

 

Hard water can contain various metal ions, including Ag⁺, Ca²⁺, Mg²⁺, and Fe³⁺. In order to fully determine the hardness of the water, each ion must be isolated separately. When a particular ion species, an analyte, needs to be isolated, it is possible to use the tendency of that ion to form an insoluble compound by a precipitation reaction.

 

When the analyte ion is formed into a precipitate, it can be collected through a process called gravimetric analysis, during which the precipitate is isolated, purified, dried, and weighed. From the mass and the known composition of the precipitate, the amount of the analyte in the original solution can be calculated stoichiometrically. When done properly, gravimetric analysis provides an extremely precise quantitative analysis of the analyte.  When completing a gravimetric analysis, an important consideration is that the analyte is completely precipitated. This can be accomplished by ensuring that the analyte acts as the limiting reactant in the precipitation reaction.  Once the salt has precipitated, it can be collected through filtration. All of the impurities should be removed from the precipitate through washing and drying. 

 

Pre-Lab: Answer the following questions to prepare for your experiment. (5 points)

1)      What is an analyte?  What is the analyte in this experiment? (2 points)

2)      What is the limiting reactant in this lab? Justify your answer. (2 points)

 

3)      Explain the process of gravimetric analysis. (1 point)

 

Procedure

Part 1: (6 points)

Copy the link below into your browser

https://phet.colorado.edu/sims/cheerpj/soluble-salts/latest/soluble-salts.html?simulation=soluble-salts

 

1.      Under the Table Salt tab, shake the saltshaker by clicking on it and moving it up and down. Describe what happens to the solid table salt, NaCl, when it mixes with water. (1 point)

 

2.      Click Reset All. Shake the saltshaker until some of the particles are designated as Bound.

a.       How many sodium ions are designated as Dissolved? (1 point)

 

b.      How many sodium ions are designated as Bound? (1 point)

 

c.       Use the simulation to describe what bound means. (1 point)

 

3.      Click the Slightly Soluble Salts tab. Using the pull-down menu, select Mercury (II) Bromide. Slowly shake the saltshaker until some of the ions are designated as Bound. Compare how the added mercury (II) bromide is different from the previously added table salt. (1 point)

 

4.      Slowly drain some of the mixture out of the container by sliding the slider on the bottom faucet. Release the slider with some water still in the container. What happens to the ions when the drain is turned off? (1 point)

 

Part 2: (6 points)

Copy the link below into your browser

http://chemcollective.org/activities/autograded/110

 

1.      In this experiment, you are trying to determine the mass of AgNO₃ that is present in “Solution 3”. Based on what you have learned about precipitation reactions in AP Chemistry and what is present in the stockroom (distilled water, NaCl, and AgNO₃), what would be the best way to determine the amount of AgNO₃ in the dissolved solution? Justify your answer. (2 points)

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2.      Click on Solutions > Click on “Solution 3”. It will automatically appear on your workbench. Click back on the Stockroom.

 

3.      Choose the substance (either Distilled H2O, NaCl, or AgNO3) that is consistent with your answer to #1 to add directly into the Solution 3. To add it to the solution, drag it on top the solution and you will get a box asking how much to add.

 

4.      Add the substance into Solution 3 a small amount at a time (1 g or 1 mL) until the mass of the product does not change. The mass of the product of the reaction will be listed on the left-hand side in the Information Pane.

a.       Write the balanced equation for the reaction that occurred.  (1 point)

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b.      Write the net ionic equation. (1 point)

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c.       Calculate the moles of Ag⁺ in the product. (1 point)

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d.      Calculate the mass of AgNO₃ in Solution 3. (1 point)

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Part 3: (13 points)

Answer the following questions relating to gravimetric analysis.

 

1.      If the gravimetric analysis was done in-person (not a simulation), identify 3 possible sources of experimental error. Explain how possible sources of experimental error may affect the experimental results. (2 points)

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2.      In the first of two experiments, a student is assigned the task of determining the number of moles of water in one mole of MgCl2 ⋅ n H2O. The student collects the data shown in the following table.

C:\Users\s317812\Documents\Term 1\chem\chemmmmmmmm.PNG (image)

 

a.       Explain why the student can correctly conclude that the hydrate was heated a sufficient number of times in the experiment. (1 point)

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b.      Use the data above to

i.        Calculate the total number of moles of water lost when the sample was heated (1 point)

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ii.      Determine the formula of the hydrated compound. (2 points)

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c.       A different student only heats the hydrate one time and claims that this will have no effect on the calculated mass of the MgCl₂. Do you agree or disagree with this student? Justify your answer. (1 point)

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3.      In the second experiment, a student is given 2.94 g of a mixture containing anhydrous MgCl2 and KNO3. To determine the percentage by mass of MgCl2 in the mixture, the student uses excess AgNO3 (aq) to precipitate the chloride ion as AgCl (s).

a.       Starting with the 2.94 g sample of the mixture dissolved in water, briefly describe the steps necessary to quantitatively determine the mass of the AgCl precipitate. (2 points)

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b.      The student determines the mass of the AgCl precipitate to be 5.48 g. On the basis of this information, answer each of the following.

                             i.      Write the balanced equation for this reaction. (1 point)

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                           ii.      Calculate the number of moles of MgCl2 in the original mixture. (2 points)

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                         iii.      Calculate the percent by mass of MgCl₂ in the original mixture. (1 point)

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