Determination of Weight

The objective of this lab is to familiarize students with the tools and equipment used in basic scientific analysis and research. The lab focuses on developing skills in measurement, problem-solving, and recording of results. Additionally, it aims to teach students safe practices in the chemical laboratory and the proper use of exponential notation and significant figures. The lab also introduces students to the measurement of density for liquids and solids.

The materials required for this lab include different types of balances such as the analytical balance, triple beam balance, and top loading balance, as well as forceps and various weighing objects.

The lab begins by discussing the importance of determining the weight (mass) of objects or samples in a chemical laboratory. The three types of balances used in the lab are explained: the analytical balance, the triple beam balance, and the top loading balance.

The analytical balance is a single pan balance that can weigh objects to the nearest 0.0001 gram. It is expensive and delicate, requiring careful handling. The lab provides instructions for the general care of the analytical balance, including avoiding air currents, not overloading the balance, not weighing chemicals or moist objects directly on the pan, and not placing hot objects on the pan. It also emphasizes the use of tongs or forceps for handling objects and the need to remove all objects and clean up any spills after weighing. Students are advised to report any issues with the balance to the instructor or lab assistant.

The triple beam balance is introduced as a balance with three weight beams and an indicating pointer. It is used to establish the weight of an object by positioning weights along the beams until balance is achieved. The lab provides a procedure for using the triple beam balance, including checking the rest point, placing the object on the platform, adjusting the weights on the beams, and recording the weight.

The top loading balance is described as one of the easiest and quickest balances to use. It involves placing the object on the platform and reading the weight directly on the optical display. The balance should be level and may need to be zeroed before weighing.

The lab also includes a section on error analysis of experimental data. It defines determinant or systematic errors as correctable errors and random errors as normal fluctuations in measurements beyond the observer's control. The concepts of mean value, deviation, mean deviation, precision, accuracy, and relative deviation are explained. The lab provides examples and procedures for calculating mean value, deviation, and mean deviation. It also introduces the criterion for dropping outliers if their deviation from the mean is greater than four times the mean deviation of the other observed values.

By conducting the lab activities using different balances and analyzing the data with error analysis techniques, students develop skills in using laboratory equipment, making accurate measurements, and evaluating experimental results.

Objective

·         Use the tools and equipment necessary for basic scientific analysis and research

·         Utilize measurement and problem-solving techniques in scientific calculations

·         Use standard glassware and instruments to manipulate and measure chemical quantities

·         Record the results of investigation through writing

·         Demonstrate safe practices in the Chemical Laboratory

·         Demonstrate the proper use of Exponential Notation and Significant Figures

·         Demonstrate how to measure the Density of Liquids and Solids

·         Use scientific reasoning to evaluate physical and natural phenomena

·         Identify the unifying themes of the scientific field of study

Materials

·         Analytical Balance

·         Triple Beam Balance

·         Top Loading Balance

·         Forceps

·         Various Weighing Objects

Discussion

Determining the weight (mass) of objects or samples is a frequent and crucial task in chemical laboratories. To accomplish this, it is necessary to use an appropriate balance. This experiment involves the use of three different types of balances: the analytical balance, the triple beam balance, and the top loading balance. Throughout this course, precise quantitative values are often needed for various experimental procedures. In such cases, the analytical balance will be utilized for accurate weight determinations.

Types of Balances

The Analytical Balance

The laboratory will utilize single pan analytical balances that have a weighing precision of up to 0.0001 gram in various situations. These balances are both fragile and costly. Figure 1 illustrates the main components of the single pan analytical balance. Your laboratory instructor will demonstrate the correct technique for operating the specific balance model in your lab. The following guidelines are provided for the general maintenance of the analytical balance:

1.       Always close the balance case before reading the weight. This prevents any disruption caused by air currents.

2.       Avoid overloading the balance. Most analytical balances can hold a maximum load of 160 g.

3.       Never directly weigh any chemicals or damp objects on the balance pan.

4.       Refrain from leaning on or jarring the balance table, as this can affect your balance's accuracy and that of others nearby.

5.       Avoid placing hot objects on the balance pan. The weight of a warm object will be inaccurate due to convection currents created by the rising heated air.

6.       Whenever possible, use tongs or forceps to handle objects being weighed. For round objects like weighing bottles, fingers may be used, but take care to prevent weight changes when placing an object on the balance pan. Avoid holding objects longer than necessary.

7.       After weighing, remove all objects from the balance and close the case.

8.       Clean any spills on the pan or within the balance case. Remove any towels or materials from the table or beneath the balance.

9.       Remove all weights and reset the balance to zero.

10.    Respect others' rights and do not monopolize the balance.

11.    If the balance is malfunctioning, immediately report the issue to your instructor or lab assistant.

Maintaining the balance in proper condition is crucial to ensure accurate work. Abusing the balance will compromise the accuracy of results and affect others as well. Failure to comply with the rules for the care and use of the balance will impact your lab procedure grade. Extensive damage may require you to bear the repair costs.

To build confidence in your ability to rapidly and accurately weigh objects while caring for the delicate balance, your instructor will provide an object of "unknown" weight. Under supervision, you will conduct a test weighing, aiming for a difference of no more than 0.0002 grams from the known correct weight. Each student is expected to achieve this level of accuracy within 3-4 minutes before proceeding to further experiments.

For most cases, you will weigh objects on your analytical balance using the "difference" method. This method is necessary for chemicals that experience weight changes when exposed to the atmosphere. For instance, let's consider the weight determination of a sample of powdered compound. The powder is contained in a small bottle, often referred to as a weighing bottle. Transfer the estimated amount of material needed from the bottle to a suitable receiver and weigh the bottle again.

Data:

First weighing (bottle plus compound)

10.7384 g

Second weighing (after sample is removed) Difference

10.4001 g

Weight of sample removed

0.3383 g

Procedure - Single Analytical Balance

1.       Ensure that the balance is level, the pan is clean, and the chamber doors are closed.

2.       Set all weight control knobs to zero and verify the balance's zero point by completely releasing the pan. Typically, adjusting the zero point at the beginning of the session should be adequate, but it is recommended to check it before each measurement.

3.       Keep the balance locked, place the object to be weighed on the pan, and close the door.

4.       Partially release the pan and adjust the weight control knobs accordingly.

5.       Completely release the pan, make any necessary final weight adjustments, and record the final weight.

6.       Lock the balance, reset the weight control knobs to zero, remove the object from the pan, and close the door.

The Triple Beam Balance

The main components of the triple beam balance include the platform for placing objects, three weight beams, and the indicator pointer and zero scale (refer to Figure 2). To determine the weight of an object, one must adjust the weights along the beams until equilibrium is reached. Specifically, the movable weights should only be placed in the notches of the middle and rear beams. As for the front beam, the sliding weight can be positioned anywhere along the graduated section of the beam.

Procedure - Triple Beam Balance

1.       Ensure that the balance is at its resting position by verifying that the platform is empty and all beam weights are set to zero. While it is preferable for the rest point to be at zero, it is not strictly necessary. If needed, adjust the screw-type weight located beneath the balance platform.

2.       Position the object that needs to be weighed onto the platform.

3.       Gradually move the weight on the middle beam one notch at a time until the balance pointer deflects and remains below the zero mark. Then, move the weight back one notch. Repeat this process with the weight on the rear beam.

4.       Modify the sliding weight on the front beam until the balance pointer swings equally above and below the zero point. If the balance was not initially set to zero, ensure that the pointer returns to its initial rest point.

5.       Record the weight as the sum of the weights on all three beams.

6.       Restore the weights to their original positions.

Top Loading Balance

In the latest models of top-loading balances, this particular type of balance stands out as remarkably user-friendly and efficient (refer to Figure 3). The process is incredibly straightforward: all you need to do is position the object on the platform and obtain the weight instantly from the optical display. It is important to ensure that the balance is level, and in certain instances, it may be necessary to zero it prior to weighing.

Procedure – Top Loading Balance

Retrieve a metallic item from the laboratory instructor or assistant and document its unidentified number in your notebook. Employ the triple beam balance, the top loading balance, and the analytical balance to ascertain the weight of the metal object. Take note of the measurements obtained for each weight using all three balances.

Error Analysis of Experimental Data

Having a comprehensive grasp of the definitions, procedures, and interpretations involved in the collection of experimental data allows scientists to effectively assess a significant amount of data that would otherwise lack meaning. The subsequent definitions aim to aid students in comprehending various aspects related to error analysis of a dataset:

a.       Systematic errors, also known as determinant errors, can be rectified.

b.       Random errors occur in all measurements and are beyond the control of the observer. They represent normal fluctuations in recorded observations.

c.        The mean value (or average) is obtained by dividing the sum of observed values by the number of observations.

d.       Deviation refers to the difference between an observed value and the mean value.

e.       Mean deviation is calculated by summing the absolute deviations and dividing by the number of observations.

f.         Precision denotes the reproducibility of a physical measurement.

g.       Accuracy represents the disparity between the observed value and the "true" value.

h.       Relative deviation is the mean deviation divided by the mean value. It can be expressed in various ways, and several examples are provided below:

Expressed as a percentage (%):

Expressed as part per thousand (ppt):

An example of the procedure for obtaining the mean value, deviation and mean deviation are given below.

Observed Values Deviations

Deviations

1.012

0.001

1.010

0.001

1.010

0.001

3 / 3.032

3 / 0.003

Mean Value = 1.011

Mean Deviation = 0.001

If the deviation of an observation from the mean exceeds 4 times the mean deviation of the other observed values, it will be considered an outlier and discarded. This criterion for rejection is valid when a minimum of four observations, including the questionable one, have been recorded.

Report Sheet for Determination of Weight

Name_______________________________________                          Date___________________________

Lab / Section_________________________________

Unknown Number________________

Review Questions

1.       What is the objective of this lab?

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2.       List three tools or equipment used for basic scientific analysis and research.

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3.       How do you demonstrate safe practices in the chemical laboratory?

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4.       Explain the difference between the analytical balance, triple beam balance, and top loading balance.

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5.       What precautions should be taken when using the analytical balance?

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6.       Why should you not weigh chemicals or moist objects directly on the balance pan?

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7.       Why is it important to handle objects with tongs or forceps when weighing?

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8.       What is the purpose of the "difference" method when weighing objects on the analytical balance?

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9.       Describe the procedure for using the analytical balance.

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10.    What are the major features of the triple beam balance?

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11.    How do you establish the weight of an object using the triple beam balance?

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12.    Explain the steps involved in using the triple beam balance.

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13.    What are the advantages of using a top loading balance?

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14.    How do you determine the weight of an object using a top loading balance?

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15.    What precautions should be taken when using a top loading balance?

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16.    Define determinant or systematic errors and provide an example.

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17.    What are random errors and why are they beyond the control of the observer?

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18.    How do you calculate the mean value and mean deviation of a set of observed values?

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19.    Explain the difference between precision and accuracy in a physical measurement.

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20.    How do you calculate the relative deviation of an observed value?

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