In your chemistry lab you will be expected to know how to interpret and
graph data found from your experiments. You will also use these graphs to help
you find missing information, in fact a lot of your labs will require
you to determine several unknowns. So let's review the anatomy of
a graph. There are two axes, the x-axis which is
horizontal and the y-axis which is vertical.
The x-axis is known as the independent variable
because this is the variable you are able to control in an experiment.
While the y-axis is known as the dependent variable
because its change is dependent on the independent variable. For example
if we have temperature as our dependent variable
and time as the independent variable and we were boiling a substance
we are able to control how long we boil the substance
but the temperature is dependent on how much time we allow the substance
to boil for. We would also see that the y and x axes will always be labeled and
must show the unit of that measurement if possible.
Next there will always be a title that shows the y-axis versus the
x-axis. The majority of your graphs will be linear and you'll notice that
your data points won't perfectly align to form a perfect line.
This is why we use something known as the best fit line,
which is a straight line that best approximates
the given set of data points. This line may pass through some of the points
none of the points or all of the points, this of course depends on your data.
Luckily most graphing software will create the best fit line
for you. You may also see the best fit line referred to as the trendline.
You will also need to understand how to interpret the equation
of this line, so let's go over the equation
of a line. Which is y equals mx plus b where m refers to the slope and b refers
to the y-intercept. The y-intercept is a point found on the
y-axis. The slope is the change of y over the
change of x ,think of this as a rate. For example,
if we had this graph that showed the x-axis as the volume
and the y-axis as mass then the slope would show the change in mass over the
change in volume. In this example the slope is actually
our density. Since the formula for density is mass
divided by volume. The slope is very useful because it will
help you determine any unknown values. If you needed to find the slope from two
points you would use this formula. Start with labeling your two points, the
first point is x1 and y1, the second point is x2 and y2.
We can plug in our labeled values into the formula and subtract
first then divide to get our slope. Typically in Chemistry you won't be
asked to find the slope by hand, you would just use the coefficient in
front of the x in your trendline. Note always make sure
that your graph has a title, each axis is labeled with the units, the best fit
line is shown and the equation for your trendline is shown.
Do this to avoid losing any points on your lab reports.
There are some chemistry formulas that can be written to represent
the equation of a line. Here's an example of this, where the ln of P
is y, this portion is the slope, our x is what is in parentheses and our
y-intercept is this. Let's say you were asked to find the
heat of vaporization but only were given this data.
This is where graphing and determining the equation of your trendline
comes in. Since we said that y is equal to
ln of P which is your vapor pressure and x is equal to 1 divided by T which is
the temperature, we first need to change each one of
these values. All temperatures must be in this form, where we will
divide one by each temperature and we will take the ln
of each vapor pressure. When we do this and have the correct
values we can graph this data and add a trendline. Now that we have our
equation of the trendline we can use this equation to find the
heat of vaporization. Remember what each part of this equation
actually refers to. If we are looking for the heat of
vaporization which is this we can use the slope and set the slope
equal to this entire term. Our slope would have the unit of kelvin
since the x-axis was referring to temperature.
Next R is a known constant which is this value.
We can plug this in and solve for the heat of vaporization
by multiplying both sides by this value and these units cancel.
Divide both sides by negative 1 to get the heat of vaporization by itself. The
typical units are kilojoules so we can divide by a thousand to get our answer
in the correct units. That was just one of the many different
types of questions you will see that involve graphing. To
see what other types of graphing questions chemistry covers, click the
link in the description once you've completed all those practice
problems come back to this playlist to see the
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