Lab Report on Stoichiometry of a Metal Complex

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Discussion Questions

  1. What was the purpose of this experiment?
  2. What is the hypothetical equation for the reaction of Fe2+ and ortho-phenanthroline?
  3. When preparing the samples, the amount of stock B (Fe+2) was constant but the amount of
    ortho-phenanthroline increased.
  • Looking at the picture below, what do you notice about the colors of the samples as you go from #1 to #8 ?
  • How does this observation compare with what was expected?
  • How was this observation evident on the graph?
  • How can you explain what happens to the absorbance of samples #2 – #5?
  • How can you explain what happens to the absorbance for the last 3 samples?
Amount of
ortho-phenanthroline increased with the amount of stock B (Fe+2) remaining constant
  1. The “n” value is the mole ratio for the balanced equation. Mole ratios in a balanced equation
    are whole number values (i.e integers). Write the full balanced equation for the reaction
    between Fe+2 and ortho-phenanthroline using in your result for “n”.
  1. The true result for n = 3. Calculate the % error. Compare your experimental result to this value and comment on the comparison.

% error = [(𝑎𝑐𝑡𝑢𝑎𝑙 𝑣𝑎𝑙𝑢𝑒−𝑒𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑎𝑙 𝑣𝑎𝑙𝑢𝑒)/ 𝑎𝑐𝑡𝑢𝑎𝑙 𝑣𝑎𝑙𝑢𝑒] x 100

  1. Discuss 3 possible sources of experimental error and how the errors would affect the results.

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Final Calculations

You should complete and submit all of the following calculations.  Remember, a complete calculations section includes:

  • Identify what you are calculating
  • The equation used for the calculation
  • The equation filled in with the data (including units)
  • The final result/answer for the calculation (with proper units and sig figs)

For this experiment, you can show your worked out calculations in the space provided or on a separate page.

This first set of calculations deals with the Fe+2 solutions used in the experiment.

Stock A Molarity:  (Note: 0.XXXX grams comes from the data pic for grams of ferrous Av

MA =

Stock B Molarity:  (note:  VA = 25.00 mL (volume of the pipet) and VB = 250.0 mL (volume of the volumetric flask)

MB =

Millimoles of Fe+2 in solutions # 1 – 8:  For each solution 10.00 mL of the Stock B solution was pipetted into each flask.

Millimoles of Fe+2  =  (10.00 mL) x (MB)

This set of calculations deals with the information from the graph.

Average Absorbance for the Horizontal Data:  Determine the average value for the absorbances of the 3 horizontal data points

Average Horizontal Absorbance =

Intersection point of the graph.  You should notice on the graph that if a horizontal line is drawn through the horizontal points, it will intersect the increasing trendline.  You can calculate the millimoles of ortho-phenanthroline that corresponds to that intersection point by using the y = m x + b trendline equation from the graph. 


  • y = your average horizontal absorbance
  • m = slope from the trendline
  • b = y intercept from the trend line

You can solve for the “x” value – this is the millimoles of ortho-phenanthroline at the intersection

y = m x + b“n” value – this is the ratio of the millimoles of ortho-phenanthroline at the intersection point to the millimoles of Fe+2 in the solutions.

“n’” = Millimoles of ortho-phenanthroline at the the intersection point / millimoles of Fe+2 in each solution

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General Graph Format

  • The graph should be a full page (8 ½ x 11)
  • The plotted points should take up most of the page (adjust x or y axis scaling if necessary).
  • Clearly show all data points
  • Make sure to label both axes with the variable and units
  • Title the graph correctly.  The title for the graph is NOT the title of the experiment.  The title of the graph is “y-variable” vs. “x-variable”… Fill in the proper labels for the x & y values.
  • Don’t connect the dots when a slope and intercept are determined from a best fit line.  Use excel to put in a trendline.

HOW TO DRAW THE GRAPH IN EXCEL (2016) For The Stoichiometry Of A Metal Complex Experiment

Note:  This graph will have 2 sets of data plotted and one best fit line with the y = m x + b equation

  • Type in the data/calcs from your worksheet table for solutions # 2 – 8; You will NOT plot solution #1 and do not include units or labels, just the numbers.
    • first column = X axis (millimoles of ortho-phenanthroline) 
    • second column = Y axis (absorbance)
  • Highlight only the x & y data for the increasing data (solutions # 2 – 5) & click on the INSERT tab along the top
  • In the CHARTS box, choose SCATTER (with only markers – top left) for the chart type. This will automatically show the graph for the 4 data points that were highlighted.
  • Double click on the [chart title] and re-type the actual title for this graph. (Remember the title is the (y vs x) actual varible labels for this graph)
  • Click inside the graphed area and then click on the [+] at the upper right corner of the graph (this allows you to add chart elements).  Be sure the following 5 items are checked:
    • Axes
    • Axis titles
    • Chart title
    • Gridlines
    • Legend
  • For AXIS TITLES – this puts generic axis titles for both the x & y axes
  • Double click on each [Axis title] and re-type the actual axis label into each.
  • Next, you will add in a second set of data.  
  • Right-click inside the middle of your graph and choose [Select Data] then [Add]
  • Fill in the [Series Name] as “Horizontal”
  • For the [Series X values], click on the red arrow to the right, highlight the millimoles for solutions # 6 – 8, click on the red arrow to get back to [Edit Series] 
  • Now for the [Series Y values] click on the red arrow to the right, and highlight the 3 Absorbance values for solutions # 6 – 8,  then click [OK] to finish the [Edit Series]
  • You should see 2 series of data listed – Series 1 should be renamed.  Click on Series 1 to highlight it, and then click on [Edit]; you can then type “Increasing” into the name box; then click [OK] to close out the Edit box and then [OK] again to close out the select data source box
  • This should show a second set of data points in a different color/shape than the first full set of data and a legend that identifies the increasing and horizontal data
  • Now to add in the best-fit trendline for that first set of data (the increasing points).  Right-Click on any one of the 1st  set of increasing data points and all of those 4 points should be highlighted.  Now choose ADD TRENDLINE.
  • The FORMAT TRENDLINE BOX should open up on the right.  Choose and fill in the following:
    • Linear
    • Forward 0.010 periods
    • Backward 0.005 periods
    • Display equation
    • Click on the [X] at top right when you are done.  This will close out the Format Trendline Box
  • This will show and extend the best-fit line and the equation for y = mx + b should appear on the graph.  You can move the equation if it overlaps data or a gridline..
  • Finally, to make the graph a full page, you can [move chart].  On the far top right, click on MOVE CHART (location) and choose New Sheet and click [OK].  Or you can right click in the white area to the right of the graph and get to [move chart] and choose “New Sheet”.
  • Save the graph as a pdf so that it can be uploaded to BB.

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