Measurement

CH104  Introductory Chemistry Laboratory

Higginbotham

Objectives:

• To become familiar with measurement of mass, volume, length, and density.
• To work with metric units and increase understanding of them.
• To increase appreciation for careful and considered measurement.

Introduction:  Scientists rely on observations to categorize and understand the materials (or organisms) they are working with.  While some very valuable observations are qualitative, many observations are quantitative.  Therefore it is worth taking some time to consider our use of measurements and numbers.  Good measurements are key to drawing appropriate conclusions from experiments.  So today we will consider some of the ideas behind good measurement and then we will develop skills in making measurements of the type that are most valuable to chemists. 

In the chemistry lab, most often we make quantitative measurements of volume and mass.  We also occasionally measure length. 

As you work, recall the importance of observation in determining the identity of substances.  Last week you used mass and volume measurements to find density.  In the future we will continue to use these types of measurements as we observe and explain chemical reactions.  Strive to be both accurate (getting the measurement right) and precise (reporting the measurement as specifically as you can with the tool you are using).

Length  (base unit is the meter)

Obtain a meter stick and a ruler. 

1.  Identify something in the room which would most appropriately be measured by the meter stick (a relatively large object).  Take a measurement of this object and then record your measurement to the nearest centimeter, with an accurate description of the object, below:

Object:                                                               Length (unit=meters):

2.  Identify an object in the room which is more appropriately measured in centimeters (cm), identify it and provide its length below:

Object:                                                              Length (unit=centimeters):

3.  Convert the measurement you took in meters to centimeters, considering 1.000 m = 100.0 cm.  Show your work, and keep every digit you initially measured as you report your answer:

4.  Convert the measurement you took in centimeters into a value expressed in meters.  Again, keep any measured digit but don't report any digits that you didn't measure:

Volume  (base unit is the liter)

Now put a sample of liquid water from the faucet into one of the flasks provided in the back of the room.  It is not critical that you get any particular volume, although you will want a volume that is easy to work with.  Just get some water into the flask.

5.  Is your flask an Erlenmeyer flask, a beaker, a Florence flask, or a graduated cylinder?

6.  According to the marks on the flask, what is the approximate volume?  Label your measurement in liters or milliliters, and provide at least 2 digits in your answer:

7.  Transfer the liquid into a graduated cylinder, which intended to be used for accurately measuring volume.  Determine the volume again, according to the graduated cylinder.  As you record the volume, report all the numbers you actually know plus one digit which you must guess at.  Be sure to label your volume in liters or milliliters:

8.  Based on the similarities or differences between the volumes you recorded using the flask vs. the graduated cylinder, which is the more appropriate tool for actually making careful measurements?

 

Some glassware, though marked with approximate volumes, is not suitable for making accurate or precise measurements.  Always use the best tool you have available to you when you measure volume.

Mass (base unit is the gram)

Collect a sample of 20 medicine tablets.  You may choose aspirin, acetaminophen or ibuprofen, but all the tablets should be the same type.

Look at the dosage information on the bottle.  Based on this information, how much do you think the group of 20 tablets will weigh, in grams?

Weigh the collection of tablets and record the mass here.  Remember to include the measured number and the unit label:

Weigh one tablet and record its mass here:

Is the single tablet exactly 1/20 of the collection of tablets, by mass? 

Does each tablet appear to weigh the same as the reported drug dose on the bottle?  If not, speculate about why it does not:

Conceptualizing amounts (mass, volume, length) using metric measurements

The following questions are intended to get you to start thinking intuitively about metric measurements.  If you use your best judgment I will not penalize you for answers that are somewhat wrong.  On the other hand, think about your answers and discuss them with your lab partners so that you begin to really get to know the metric units we are considering.

10.  What familiar object might have a mass of approximately:

            1 gram?

            100 grams?

            1 Kg?

11.  What do you think is the best estimate of the volume of a small (12 oz) cup of coffee?

            a quarter of a liter

            50 milliliters

            1 liter

12.  Which measurement unit seems most practical for injected drugs?

            microliter

            milliliter

            liter

 Density

Density is defined as mass divided by volume.  Chemists usually work with grams divided by milliliters.  Density is an intrinsic property of a substance that takes into account both mass and volume, so it is a measured value that does not change no matter how large or small a sample one has.  Because it is not dependent on sample size, it is a valuable measured physical property that can be used to identify substances.

The density of wine and beer are a result of their compositions.  These mixtures are composed mostly of water, ethanol ("alcohol") and sugars. Different types of wine and beer have characteristic densities that are often measured by people and companies that produce them.   

Water has a density of about 1.0 grams/milliliter.  Ethanol has a density that is a bit less at 0.79 g/mL.  Sugars in these drinks increase the density.  So each individual recipe produces a beverage with a certain amount of each...and therefore a characteristic density.  A stout beer, because it contains more sugars and is not very high in alcohol will have a higher density than an IPA.

13.  Five samples of ethanol/water mixtures have been set up for you to measure.  Two have been labeled to show the percent of alcohol.  Determine the densities of each of these solutions, then plot the density vs. % alcohol on a graph.  Draw a line to show the relationship between density and % alcohol based on your data.

Determine the amount of alcohol in each of the other three samples by determining their densities experimentally and using the graph.

What is the percent alcohol in each of the other samples? 

Sample A contains:

Sample B contains:

 Sample C contains:

The principle ingredients in soda pop are water and high fructose corn syrup, with flavorings and preservatives present in smaller quantities.  Diet sodas contain water and flavorings and usually a very small quantity of the sweeter sugar substitutes. 

14.  Based on this information, describe how you might be able to tell the difference between diet soda and regular soda experimentally and without consulting the label or opening the cans.  You may use a bucket of water and a sealed can of each type of soda.