### Basic Graphing in Excel

• Entering and Formatting the Data in Excel
• Creating the Initial Scatter Plot
• Creating a Scatter Plot of Titration Data
• Changing the Scatter Plot to a Line Graph
• #### Introduction

Beer's Law states that there is a linear relationship between concentration of a colored compound in solution and the light absorption of the solution. This fact can be used to calculate the concentration of unknown solutions, given their absorption readings. First, a series of solutions of known concentration are tested for their absorption level. Next, a scatter plot is made of this empirical data.

#### Entering and Formatting the Data in Excel

Open Excel. On Unity/Eos computers, the program will be located on the Application Launcher. On other computers, it will probably be located under the Start Menu.

Your data will go in the first two columns in the spreadsheet (see Figure 1a).

• Title the spreadsheet page in cell A1
• Label Column A as the concentration of the known solutions in cell A3
• Label Column Bas the absorption readings for each of the solutions in cell B3
• Begin by formatting the spreadsheet cells so the appropriate number of decimal places are displayed (see Figure 1a).
• Click and drag over the range of cells that will hold the concentration data (A5 through A10 for the sample data)
• Choose Format > Cells... (this is shorthand for choosing Cells... from the Format menu at the top of the Excel window)
• Click on the Number tab
• Under Category choose Number and set Decimal places to 5
• Click OK
• Repeat for the absorbance data column (B5 through B10 for the sample data), setting the decimal places to 4
• Figure 1a

If you do not have your own data, you can copy the data seen in Figure 1b.

• Enter your data below the column titles
• You can also place the absorption readings for the unknown solutions below the other data.
• Figure 1b.

The concentration data is probably better expressed in scientific notation.

• Highlight the concentration data and choose Format > Cells...
• Choose the Scientific Category and set the Decimal places to 2

•

The last step before creating the graph is to choose the data you want to graph.

• Highlight the data in both the concentration and absorbance columns (but not the unknown data)

•

This is shown in Figure 2.

Figure 2.

#### Creating the Initial Scatter Plot

With the data you want graphed highlighted, start the chart wizard
• Choose the Chart Wizard icon from the tool bar (Figure 3). If the Chart Wizard is not visible, you can also choose Insert > Chart...
• Figure 3.

The first dialogue of the wizard comes up

• Choose XY (Scatter) and the unconnected points icon for the Chart sub-type (Figure 4a)
• Figure 4a.

• Click Next >
•
The Data Range box should reflect the data you highlighted in the spreadsheet. The Series option should be set to Columns, which is how your data is organized (see Figure 4b).

Figure 4b.

• Click Next >
• The next dialogue in the wizard is where you label your chart (Figure 4c)
• Enter Beer's Law for the Chart Title
• Enter Concentration (M) for the Value X Axis
• Enter Absorbance for the Value Y Axis
• Figure 4c.

• Click on the Legend tab
• Click off the Show Legend option (Figure 4d)
• Figure 4d.

• Click Next >
•
Keep the chart as an object in the current sheet (Figure 4e). Note: Your current sheet is probably named with the default name of "Sheet 1".  Alternatively chose As new sheet to prepare for printing as a single sheet the size of the sheet.

Figure 4e.

• Click Finish
• The initial scatter plot is now finished and should appear on the same spreadsheet page as your original data. Your chart should look like Figure 5. A few items of note:
• Your data should look as though it falls along a linear path
• Horizontal reference lines were automatically placed in your chart, along with a gray background
• Your chart is highlighted with square 'handles' on the corners. When your chart is highlighted, a special Chart floating palette should also appear, as is seen in Figure 5. Note: If the Chart floating palette does not appear, go to Tools>Customize..., click on the Toolbars tab, and then click on the Chart checkbox. If it still doesn't show up as a floating palette, it may be 'docked' on one of your tool bars at the top of the Excel window.

•

With your graph highlighted, you can click and drag the chart to a wherever you would like it located on the spreadsheet page. Grabbing one of the four corner handles allows you to resize the graph. Note: the graph will automatically adjust a number of chart properties as you resize the graph, including the font size of the text in the graph. You may need to go back and alter these properties. At the end of the first part of this tutorial, you will learn how to do this.

Figure 5.

To take advantage of your newly created Beer's Law plot, Excel can calculate and plot a least squares fit trendline for the graph you have just created.  The equation will be in point slope form, y = mx + b.

• Right Click on any data point
• This will display the popup menu (Figure 6a)
• Click on  Add Trendline.

• Figure 6a

from the Trend/Regresion type on the Type tab-card:

• Click on Linear (Figure 6b)

•

Figure 6b
• Click on the Options tab
• Click (check) the Display equation on chart  (Figure 6c)
• Click (check) the Display R-squared value on chart  (Figure 6c)
• Figure 6c

• Click on OK
• Your least squares fit is now complete (Figure 6d).  You can click and drag on the label in the box and format the label to increace the font size.  Additionally, by clicking, holding, and dragging on the hashed box around the text you can relocate it anywhere on the graph.

Now copy the equation by highlighting the text and using the Ctrl +C keys (alternatively click on Edit then Copy on the Toolbar at the top of the Excel screen).

Paste it on your spreadsheet by selecting the spreadsheet, selecting a cell, and using the Ctrl +V keys (alternatively click on Edit then Paste on the Toolbar at the top of the Excel screen).

Figure 6d

#### Creating an Initial Scatter Plot of Titration Data

In this next part of the tutorial, we will work with another set of data. In this case, it is of a strong acid-strong base titration (see Figure 10). With this titration, a strong base (NaOH) of known concentration is added to a strong acid (also of known concentration, in this case). As the strong base is added to solution, its OH- ions bind with the free H+ions of the acid. An equivalence point is reached when there are no free OH- nor H+ ions in the solution. This equivalence point can be found with a color indicator in the solution or through a pH titration curve. This part of the tutorial will show you how to do the latter.

Note that there should be two columns of data in your spreadsheet:

Column A: mL of 0.1 M NaOH added

Column B: pH of the 0.1 M HCl / 0.1M NaOH mixture

• Using a new page in the spreadsheet, enter your titration data. If you do not have your own data, use the data shown in Figure 7.
• Go to the Data Input Tutorial if you need hints on formatting the cells to the proper number of decimal places
• Figure 7.

Now, create a scatter plot of titration data, just as you did with the Beer's Law plot.(Figure 6d).

• Highlight the titration data and the Column headers
• Click on the Chart wizard icon
• Choose XY (Scatter) and the scatter Chart sub-type
• Figure 8.

Continue through steps 2 through 4 of the Chart wizard:

• The defaults for step 2 should be fine if you properly highlighted the data
• In step 3 enter the chart title and x and y axis labels andturn off the legend
• In step 4, leave as an object in the current page
• The resulting plot should look like Figure 9:

Figure 9.

#### Changing the Scatter Plot to a Line Graph

All of the points of the titration data can be connected to form a smooth curve. With this approach, the curve is guaranteed to go through all of the data points. This is both good and bad. This option can be used if you have only one pH reading per amount of NaOH added. If you have multiple pH readings for each amount added on the scatter plot, you will not end up with a smooth curve. To change the scatter plot is a (smoothed) line graph (Figure 10):
• Choose Chart > Chart Type...
• Select the Scatter connected by smooth lines Chart subtype
• Figure 10.

The result should look like Figure 11:

Figure 11.

This smooth, connected curve helps locate where the steep part of the curve passes through pH 7.