Stoichiometry of Barium Chromate Formation

References:  Chemistry, 4^th ed, by J. McMurry and R. Fay. Prentice Hall, 2004, Sections 3.6, 3.7, 4.3, 4.4

Purpose: 

1. To confirm the stoichiometry of the precipitation reaction of sodium chromate with barium chloride
2. To use the reaction to determine the concentration of a solution of barium chloride.

Method:  Precipitation reactions are useful in quantitative analysis. For example the concentration of a AgNO₃ solution can be determined by adding an excess amount of NaCl and weighing the resulting AgCl precipitate:

NaCl(aq) + AgNO₃(aq)  ®  AgCl(s) + NaNO₃(aq)

The method works only if AgNO₃ is the limiting reagent and NaCl is the reagent in excess, otherwise not all of the silver ion will precipitate.

Example: To each of four 10. mL samples of 0.50 M AgNO₃(aq) are added 4, 6, 10 and 12 mL volumes of 0.60 M NaCl(aq). The  AgCl(s) formed is collected, washed, dried and weighed.  The mass is then determined for each sample. The results are given below:

A graph of mmol AgCl collected versus mmol NaCl added is shown in the Figure below.

Figure 1: Mmol of AgCl Precipitate versus Mmol of NaCl Added.

Please note: M = mol/L = mmol/mL

Along line AB, NaCl is the limiting reagent, and along line BC, AgNO₃ is limiting. In those solutions where NaCl is limiting, the stoichiometry of the reaction predicts that the amount of AgCl formed is equal to the amount of NaCl added. Thus, the slope of line AB is one.  Alternatively, by measuring the slope, the stoichiometry can be determined.

In the solutions in which AgNO₃ is limiting, the amount of AgCl formed is equal to the amount of AgNO₃ initially present. The line BC is horizontal because the mmoles of AgNO₃ is the same in all of the solutions. Its y intercept is 5.0 and is equal to the mmoles of AgNO₃ initially present in the solution.  From this result, the concentration of the AgNO₃ in the original solution can be determined.

In this experiment, the stoichiometry of the reaction

will be confirmed.  In addition it will be used to determine the concentration of a solution of BaCl₂. The method will be exactly the same as described for the silver chloride reaction above.

Procedure: 

The reaction mixtures in Table 1 will be used. In each case a precipitate of BaCrO₄(s) is formed upon mixing Na₂CrO₄(aq) and BaCl₂(aq). The precipitate will be collected by filtration, washed, dried and weighed. 

In those mixtures in which BaCl₂ is the limiting reagent, the mmol of precipitate will equal the mmol of BaCl₂ originally present . These mixtures will contain an excess of Na₂CrO₄,  and the filtrate will be yellow due to the presence of the yellow chromate ion (CrO₄ ^2- ). Thus, in these mixtures, the amount of precipitate can be used to determine the concentration of the original barium chloride solution.

In those mixtures in which Na₂CrO₄ is the limiting reagent, the mmol of precipitate will equal the mmol of of Na₂CrO₄ originally present, and the filtrate will be colorless.

Table 1.

Each student will be responsible for preparing three of these solutions and for determining the mass of precipitate formed in each of the three.   The entire set of class data will then be combined and used to prepare a graph of mmole BaCrO₄ formed versus mmole of Na₂CrO₄ added.  From the graph,   the concentration of the barium chloride solution will be determined and the stoichiometric relationship between BaCrO₄  and Na₂CrO₄  verified. 

Part 1: The Precipitation:

Your instructor will assign one of the following groups of three solutions to you:

Clean three beakers (100, 150, or 250 mL) and three 250 mL Erlenmeyer flasks. Label each beaker and each flask with the number of each one of your three solutions. To each of the three labeled beakers pipet 10.0 mL of the BaCl₂ solution, and then the appropriate volume of Na₂CrO₄ solution. The best way to do this is to rinse the pipet with a little of the BaCl₂ solution, then pipet the 10.0 mL into each of the beakers.  Next, rinse the pipet with deionized water and then with a little of the Na₂CrO₄ solution.  Finally, pipet the required volumes of the Na₂CrO₄ solution into each beaker.  Rinse the pipet with water before you return it.  Record the color of each of the resulting solutions. The yellow color of the Na₂CrO₄ solution is due to the CrO₄^2- ion and a yellow product solution indicates an excess of the CrO₄^2- ion.

Stir each solution well, place it on a hot plate and heat to almost boiling. Do not boil as this may cause spattering and loss of some of the solution. This heating of the precipitate is referred to as digestion and it  causes the precipitate particles to grow in size making them more easily filterable and purer.  Allow the digestion to proceed for 15 minutes.

While the samples are digesting, label three pieces of filter paper with the numbers of your solutions. Weigh them to 0.001 g and record the masses in your notebook.  Be certain that the balance is set to zero grams before weighing.   Fold the filter paper as instructed and place in a funnel. Place the funnels in the three Erlenmeyer flasks.

When the digestion is completed, allow the precipitates to settle for a few minutes and then transfer each one completely to the corresponding filter funnels. Be sure you transfer as much of the precipitate as you can.  The yield is an important part of your grade.  Your instructor will advise you of the best way of doing this.  Save the filtrates for further testing.

Wash each precipitate with a few 5 mL portions of deionized water.  Discard this wash water.

Next, wash each precipitate with three 5 mL portions of methanol. The methanol is water soluble and will wash away the remaining water.  The methanol is volatile and will hasten the drying of the precipitate. Caution: methanol is very flammable and must not be used near an open flame.  Dispose of the methanol in the "Solvent Waste" container.

Label and weigh three watch glasses, and record the masses in your notebook.

Once all of the methanol has drained away carefully remove the precipitates and filter papers from the funnels. Place the filter papers and precipitates on the preweighed watch glasses. Spread out the precipitates and carefully place the watchglasses containing the precipitates and filter paper in your drawer.  Let the precipitates air dry for one week.  Weigh the precipitate-watchglass-filter paper combinations and record the results in your notebook.   Dispose of the barium chromate and the filter paper in the designated container or as instructed.

Part 2: The Filtrates:

The reagent in excess can be identified from the color of the filtrate. A clear yellow solution indicates excess sodium chromate, whereas a clear colorless solution is an indication of excess barium chloride. Perform procedure b), below, only if the filtrate is cloudy.

a) Observe the color of each filtrate and record your observation.

b) Add about 2 mL of each filtrate to each of two test tubes. To one of the two test tubes add about 0.5 mL of the 0.6 M Na₂CrO₄ solution and to the other add about 0.5 mL of the unknown BaCl₂ solution. If a precipitate forms upon addition of Na₂CrO₄ then BaCl₂ is in excess, alternatively, if a precipitate forms upon addition of  BaCl₂ then Na₂CrO₄ is in excess. Record your observations in the Results Section. Dispose of the filtrates in the "Aqueous Waste" container.

Part 3: Class Data

Calculate the mass of precipitate collected from each of your solutions.  Record the results in your notebook. While you are doing this, your instructor will give you a data table handout.  For each of your solutions, record the mass of precipitate and the color of the filtrate on the data table.  

Enter into your notebook the class data for each of the solutions 1 through 9.

Results: (All calculations, tables and graphs belong in the results section.)

1. Use the class data to calculate the average mass of BaCrO₄ for each solution.  There is an "average" function in Excel that can save you time.  Convert these mass values to mmol.  If you excluded some data, identify the values you did not use and justify why you did not use them.  

2.  Prepare a table containing the solution numbers, mmol of Na₂CrO₄ , mmol of BaCrO₄ and the limiting reagent for each solution. (Use the average of all class data for each solution.)   Show one sample calculation for mmol of sodium chromate and one sample calculation for the mmol of barium chromate.

3.  Use Excel to prepare a graph of the mmol of BaCrO₄ versus the mmol of Na₂CrO₄ added. Select the data for the solutions where sodium chromate was limiting and determine the slope of that part of the data using linear regression.  Show the equation of the regression line.  With little more effort, you can show both lines on your graph as in Figure 1.

4.  Use your graph to determine the concentration of the original barium chloride solution.  Show your calculation for this.

Conclusion:

Your conclusion must contain the following information, written in a well-organized format: