Chapter 1. Experiment 1: Density Determination
Introduction
Author: Steven Brown
Experiment 1: Density Determination
In the never-ending quest to improve our lives, we humans are constantly asking questions about the world around us. Some of those questions fall into the realm of chemistry. What is it? What is it made of? What does it do? How much is there? These are analysis questions. With this experiment we will begin our study of how chemists create and answer analysis questions. We will do so by considering how analysis can be applied to one of the more vexing problems facing mankind today: the effect of modern human activities on the environment.
Analysis is at the heart of Chemistry. To perform an analysis requires an understanding of the properties of the materials to be analyzed. With this experiment you will analyze a series of plastics by measuring certain properties of these materials.
In order for you to be successful in this and future labs, you will need to be able to make reliable observations and measurements. Much of this course will be devoted to developing these skills. This lab provides you the opportunity to develop skill with two types of measurement: mass measurement and volume measurement. You will also learn how to use a laboratory notebook, how to perform calculations, how to present results, and how to use a common spreadsheet program.
Educational Objectives: A student who has successfully completed this experiment will be able to
- identify unknown materials using intrinsic properties,
- properly use an analytical balance to make mass measurements,
- properly use a graduated cylinder to make volume measurements,
- identify procedural and equipment limitations that limit mass and volume measurements,
- collect data (both measurements and observations) in a lab notebook, and
- organize data into tables and graphs for presentation in a report.
- determine molarity.
Experimental Objectives: A student who performs this experiment is asked to
- determine the precision of the analytical balances available in this course,
- determine the precision of the graduated cylinders available in this course,
- determine the density of various plastic samples and
- use this information to identify unknown samples.
- the NaOH solution against a primary standard.
Overview
This experiment asks you to do the following:
As a group, determine the density of a known plastic.
Individually, determine the density of a unknown plastic.
Background
At six billion and climbing the human population is having a profound effect on our planet. We generate enormous quantities of materials and much of this ends up as waste. It is estimated that the average American creates over 1300 pounds (590 kg) of trash every year. What to do with this waste is an issue of public concern.
Plastics
Plastics make up some 9% by weight of the typical American’s trash. Most of this is in the form of used containers and packaging material. Historically this material has been used once and then sent to a landfill. Unfortunately, as the use of plastics for packaging has mushroomed, so has the demand for landfills. Recycling is now considered to be one of the best means to limit our need for new landfills.
But recycling poses a problem. Not all materials can be recycled in the same way. In prepping discarded plastics for reuse each distinct kind of plastic must be handled differently. Before plastics can be recycled they must first be separated into homogeneous piles. Such separations are a common problem in chemistry. The solution is to take advantage of the differences in properties of different materials to achieve a separation.
Measurement
Prior to the 17th century, chemical discoveries were based on observations of phenomena. Measurement was a crude and little used tool. By the early 1700's, though, careful measurement had become an accepted experimental tool and rather quickly led to the development of modern chemical theories. Today, precise measuring devices are an integral part of both research and applied science. The correct interpretation of experiments requires both precise measurements and an understanding of the error associated with those measurements.
There are a number of techniques you will need to use for this experiment. They are listed here. Details can be found from the techniques page.
- Measurement reliability
- Percent Yield
- Graphing
- Mass measurement
- Linear measurement
- Volume measurement
Techniques
The following techniques will be required for this experiment. You will need to be familiar with them to successfully perform this experiment.
Mass Measurement – you will need to accurately mass the primary standard, KHP.
Linear Measurement – you will use a Vernier caliper.
Volume Measurement – pay particular attention to the use of the buret. You should practice using the stopcock with water before adding any reagent.
Data Manipulation – for this experiment the material on basic math, measurement reliability, percent yield and graphing are all pertinent.
Mass Measurement
In a chemistry lab, the mass of an object is determined by massing with a balance. If little or no accuracy is required, this is a rather trivial procedure. If a great deal of accuracy is required, it can be quite complicated.
Some issues to consider when trying to accurately mass an object.
- A wet object is heavier than a dry object. For example, a damp beaker can easily weigh 0.01 g more than a dry one. in addition, the weight of a damp object will change with time as the water evaporates. If you place an object on a balance and notice that the weight continually drops, there is a good chance the object is wet. This could limit the accuracy of the measurement to 0.01 g or less, depending on the amount of water present.
- A dirty object is heavier than a clean object. 0.001 g of dirt is not much dirt. For lab purposes, dirt is any unaccounted-for material such as dust, old labels, pieces of glass, grease, etc. even visually small amounts of dirt can be a real problem if the object is weighed more than once. Dirt that comes off the object between weighings will adversely affect results.
- Fingerprints have weight. Fingerprints can weigh 0.001 g. Getting fingerprints on objects will invalidate the weighing at the 0.001 g level and must be avoided for accurate weighings.
- Hot items. Objects weigh less at higher temperatures. Even a moderately warm sample can weigh 0.001 g less than at room temperature. This is because the object heats the air above it, reducing the density of that air. The balance is zeroed against the room temperature air above the pan. The hot air weighs less and upsets this balance.
The balance itself must also be considered. The reliability of the balance will depend on its cleanliness. Dirt and chemicals that get inside will have unpredictable effects. Chemicals, in particular, can react with the balance, causing corrosion and reducing its life span. To ensure that our balances give you reliable masses and to preserve them for future students, the following rules govern the use of balances.
- Balances should not be moved. If you feel the urge to do so, consult a lab instructor. Every time a balance is moved it needs to be recalibrated.
- Nothing but glass or metal objects should ever be placed on the surface of a balance. Chemicals must never come in contact with the balance pan or any other part of the balance. For this reason, most of the weighing you do will be weighing by difference. In this technique, a container (beaker, weighing paper) is first weighed and then the material is added and the weight measured again. The weight of the material is the difference between the two weights.
- The balance should be cleaned after use. Chemical spills must be cleaned up immediately. If you suspect that chemicals got inside the balance, report it to an instructor so that the balance can be properly cleaned.
Weighing a solid object that holds its shape (e.g., a beaker) is straightforward. The object is placed on the balance pan and the weight measured. Other situations may be more complicated. Here are some of the more common ones.
Weighing chemicals. Since chemicals must not be put directly on the balance pan, weighing by difference must be employed. Weighing paper is used to weigh chemicals that will be transferred to another container. This is a plastic-coated paper that chemicals will not adhere to. After weighing, the entire mass can be slid into a container. First the paper is weighed. Then the paper and chemical are weighed. The weight of the paper is subtracted to obtain the weight of the chemical. Some balances allow you to tare a container. To tare is to adjust the balance to read zero with the container on it. Electronic balances can be tared with the push of a button. Another suitable container is a beaker or flask. If the chemical is to be used in a beaker or flask, it is best to make the weighing directly in the container. First weigh the container, then add the chemical and weigh again. The weight of the container is subtracted to obtain the weight of the chemical. Again, it may be possible to tare the balance with the container on the pan.
Weighing liquids. The problem is similar to that of weighing a solid chemical. The solution is the same. Weigh an empty, dry container. Add the liquid and weigh again. Subtract the weight of the container to get the weight of the liquid.
Weighing by subtraction. Sometimes we’re not sure how much of a chemical we’ll be using, but we still want an exact weighing. Weighing by subtraction addresses this problem. The maximal amount of chemical is estimated. A container is prepared with this amount of chemical plus a little extra. The container and chemical are weighed. Chemical is removed and used as needed. When done, the beaker and left over chemical are weighed. The difference is the weight of chemical used.
Procedure
This experiment requires you to work with a group. You are not expected to personally make every measurement described here. Rather, you and your group members are expected to collect a complete set of data. In addition you are to learn how to properly make mass and volume measurements. It is important that you personally make enough measurements to ensure this happens
Density of a Known Plastic.
Your group is to determine the density of a piece of plastic to the best of your ability. The tools available to you in the lab are: a milligram balance, a ruler and a caliper. The piece of plastic will be provided by your instructor.
Identify a balance to use. Examine the front panel and familiarize yourself with the controls.
Determine the mass of the plastic. Determine its volume via displacement. Also determine the volume by performing the required linear measurements and calculating the volume.
Calculate the density. Compare your results to those working with the same plastic.
Identifying Plastics using Mass and Volume Measurements.
Your instructor will provided you with an unknown piece of plastic. Using mass and volume measurements, determine its identity.
Check Your Understanding
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