CHEM 2204

Chemical Laboratory Techniques

Browsing Posts in Graphing

When using Excel to generate this graph, be sure to pay attention to the following points:

  1. When adding the trendline, right-mouse click over a data point. From the context menu, select:
    • Linear as the Trend/Regression Type
    • Set intercept 0,0
    • Display line on chart
    • Display R-squared value on chart

  2. For this graph, the slope of the trendline needs to display 3 significant figures in order to get 3 significant figures for the sample’s unknown concentration. Right-mouse click over the equation of the trend line. From the context menu, select Format Trendline label.
  3. Select Number on the left menu. In the Category menu, select Number. In the Decimal Places field, type ‘4’. Click Close.

When you prepare your computer-generated graph for next week, do NOT use the Physics software “Graph it”.
“Graph it” is a macro for Excel. You should learn to generate the graph from inputting the data in Excel.

Start by inputting the data points into Excel, select the points and click Insert > Scatter (see image below).

That’s how you start! Go to the other posts to refine your graph so that it will include everything that’s needed. Also refer to your hand-plotted graph for this exercise.

Have fun!

Many important relationships can be expressed by graphing. For the analysis of materials, a standard curve is used to determine the concentration of a substance (i.e. – the unknown sample).

The analyst first prepares samples of the substance in various known concentrations (i.e. – the standards). The method of preparation should be similar to the unknown that is being measured. The analyst subjects the standards, one at a time, to an analytical instrument and records the data pair (Sample Concentration, Instrument Response) for each standard.

The following is data collected for a set of standards. A sample of some unknown concentration gives an instrument response of 0.78 mV.

First, plot a standard curve using the following data. A graph of the instrument response is plotted on the Y-axis and the concentration on the X-axis. The known variable is always plotted on the X-axis and the measured variable is always plotted on the Y-axis. Use the standard curve to determine the unknown concentration of the sample.

Sample Concentration (g/mL) Instrument Response (mV)
0.0 0.00
10.0 0.14
20.0 0.28
30.0 0.41
40.0 0.52
50.0 0.71
65.0 0.84
75.0 1.04
80.0 1.15

How to generate an acceptable graph to determine the concentration of the unknown sample.

  1. Use the whole sheet.
  2. Label the axes. (Note: the axes have units.)
  3. Write the title of the  graph. (Note: “Y versus X”)
  4. Draw the best straight line through the points. Force the line through the origin, (0,0). (Note: Do not connect the dots.)
  5. Determine the equation of the line. (Note: For hand-drawn graphs, pick two points that the line goes through. These points may or may not be the data points. For computer-generated graphs, add the trendline and the R2 value.)
    Write the equation of the line on the graph.
  6. For computer-generated graphs, need to add minor tick or grid marks on both axes.
  7. Write the slope of the line on the graph. (Note: slope has units.)
  8. Use the equation of the best fit line to calculate the unknown concentration of the sample.
  9. Label the unknown sample point on the graph. Draw a line from this point to the y-axis. Draw a line from this point to the x-axis. These lines MUST be perpendicular to the axes.
  10. Write the concentration of the unknown sample on the graph.

Let your instructor sign your hand-drawn graph before you leave. Graph the same set of data using Excel. Print a copy of the graph and let a classmate do a written peer review on the graph. Discuss the peer review together. Fix your graph in Excel. Reprint the fixed graph and hand in BOTH Excel-generated graphs (classmate critiqued graph and the corrected graph) next week.

Next week, this is what you hand in:

  • A cover page that clearly indicates your name and the peer reviewer(s)’ names.
  • Print and label your original graph : GRAPH 1.
  • Print GRAPH 1 for each peer reviewer to review your graph. Reviewer must use a different colour pen to write comments on the graph to make your comments stand out. Call this graph “Graph 2″.
  • An Excel-generated graph with the corrections. Call this graph “Graph 3″.

Introduction:

  1. Welcome, class!
  2. Course outline
  3. Calendar
  4. Laboratory
  5. Lab orientation
  6. Grading Scheme

Review: