How to Construct a Graph                                

 

Creating and reading graphs is an essential skill in science.  This exercise will lead you through the steps of constructing graphs.

 

Here is a data table to use for practice in constructing a graph.  A student made dilutions of 0.02 M cupric sulfate in EDTA, calculated the Molarity, and then put the solution in a test tube in Spectronic 20 spectrophotometer with wavelength set at 740 nm and read the Absorbance values.

Results:

Absorbance 740 nm

Conc. Cu++

0.000

0.000 blank

0.201

.002 M

0.398

.004 M

.592

.006 M

.824

.008 M

.983

.010 M

1.203

.012 M

 

When making a line graph with points on it, follow this procedure:

  1. Obtain a piece of graph paper, draw a horizontal line (x-axis) and an intersecting vertical line (y-axis).  (can use sample on page two)
  2. The rule is that the independent variable (the manipulated variable, the one that you can control) goes on the horizontal x-axis.  The dependent variable (the responding variable, the one that results from the manipulated variable) is plotted on the vertical y-axis.  In this case, the independent variable is the concentration of copper sulfate and the dependent variable is the Absorbance.
  3. Next, look over the range (high and low numbers) of values for each variable.  Range usually goes from zero to the highest value of the variable.  The range of Absorbance, for example, goes from zero to 1.203 in this data table.  The range of Molarity goes from zero to 0.012 M.
  4. Choose a scale for each axis.  The rule is that each line on the grid increases in value by the same amount as all the other lines going in that direction (on x-axis, for example).  Put in the numerical scale.  Label the x and y axes.
  5. Plot the points.  Start with lowest values and continue to highest.
  6. After all points are plotted, draw a line through the points to show the relationship between the two variables – often it won’t be a perfect straight line through the points, but you can draw a line that is straight and comes closest to most of the points – “line of best fit.”
  7. Put title on your graph.

 

 

 

 

 

 

 

 

 

 

 

Question

Now test your ability to read your graph.  The instructor gives you an unknown, you put it into the spectrophotometer as above and get an Absorbance reading of 0.675; using your graph, you determine that the concentration of copper sulfate is _____________.

 

 

You may want to use a spreadsheet program like Excel and make use of its ability to plot the graph from your data table.  For directions on how to plot a graph using Excel, follow this link.