RDGgraph - Reading Graphing Techniques Revised GRAPHING...

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Reading: Graphing Techniques Revised 1/14/10 1 GRAPHING TECHNIQUES Mathematical relationships between variables are determined by graphing experimental data. For example, the linear relationship between the concentration and the absorption of dilute colored solutions leads to Beer’s Law (A= C l ). Once the mathematical relationship is known, experimental quantities can often be calculated from the slope or y-intercept of a graph. For example, reaction enthalpy can be determined from the slope of a plot of ln k vs. inverse T. Furthermore, extrapolation of graphical data trends can be used to find information about conditions difficult to achieve in the laboratory, such as high or low temperatures and pressures. Plotting Variables A line graph consists of two axes at right angles. The horizontal x-axis , or abscissa , is typically chosen to represent the independent variable which is intentionally manipulated by the experimenter. The vertical y-axis , or ordinate , is chosen to represent the dependent variable which changes as the independent variable is manipulated. Example: A student wishes to measure the relationship between volume and temperature of a gas. If the student purposely increases the temperature and measures the resultant changes in gas volume, he should graph the values for T on the x-axis and the resulting values for V on the y- axis. If the student reverses the experiment and purposely increases the volume of the gas and measures the resultant temperature, the variables are then reversed: V is plotted on the x-axis and T is plotted on the y-axis. Selecting a Title and Labeling the Axes The title must describe the type of reaction or system being investigated, chemicals used, and special conditions of the experiment. Examples: "Beer’s Law Plot for Co(NO 3 ) 2 (aq) at 510 nm" or "Cooling Curve for Lauric Acid/Unknown Mixture". Do not state x vs. y in the title , that is easily determined by labeling the axes. Axes must be labeled with the name (or symbol) of the variable and include the units (if any). Examples: Temperature (°C) or V (mL).
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Reading: Graphing Techniques Revised 1/14/10 2 Choosing an Appropriate Scale The scale of the graph should be chosen so the data completely fills the graph and is not restricted to one small region or corner. Examples are shown in Figure 1. Note that the scale
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RDGgraph - Reading Graphing Techniques Revised GRAPHING...

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