ACTIVITY OF WET LABS MEASURING THE MASS & VOLUME OF OBJECTS AND SUBSTANCES

Measuring the Masses of Objects and Substances
Objective
Upon completion of this laboratory exercise, students will be expected to:
⦁ accurately measure the masses of solid objects, granular solids, and liquids.
⦁ choose the appropriate type and size glassware for measuring the masses of solid objects, granular solids, and liquids.
Introduction
The accurate measurement of masses is integral to understanding the physical properties of matter because the mass of a substance is a directly related to the amount of matter in the sample being measured. Among the first and most important techniques learned in a chemistry laboratory is how to properly measure the masses of substances in a variety of forms. Choosing the right tools for such measurements ensures that the integrity of a sample will not be compromised. In this experiment, we will be using a Digital Electronic Balance (Figure 1).

Figure 1. Digital Electronic Balance. This balance measures to the nearest 0.1 g.
When using a laboratory balance, it is important to consider the following:
⦁ Make sure the balance is clean before and after using it. All balances can be damaged by liquids, and electronic balances are particularly susceptible to damage by moisture. Powders can easily work their way into crevices in the balance, causing it to give inaccurate readings. Ensuring the cleanliness of a balance may involve wiping it down with a paper towel to ensure that all liquid is removed. If granular solids are present prior to use, you may need to use a small broom or paintbrush to brush them away and dispose of them.
⦁ Make sure that the balance is tared when there is nothing on the balance. On modern electronic balances, the mass of the weighing paper or container can be set to 0.000 g (also known as tared out or zeroed out). This has the effect subtracting its mass immediately (and automatically). The sample is then placed on the weighing paper or in the container, and the balance reading is the mass of the sample. In a kitchen, it may be okay to tare the balance with a dish prior to weighing an ingredient because chances are that only one person is using it at a time. But in a chemistry lab setting, there tend to be many people performing an experiment, which means that someone else might tare the balance in between your measurements, and you will have to start over. Instead, you can zero out the balance first, then measure the mass by the difference method, which will be explained in further detail later. For either method, the resultant mass of the sample is the same and is called the tared mass of the sample.
⦁ When measuring a mass, the sample should never touch the balance. Rather, the sample should be carefully placed either into a beaker, flask, weigh boat, weigh paper, or other such container (Figure 2) in such a way as to avoid spills. This helps to keep the balance clean and avoids contamination by whatever may have come in contact with the balance before you use it.
⦁ Always measure samples at room temperature. Warm samples can heat the air around them and create convection currents; warm air rises, which can cause a measurement to be much less than the actual mass.
⦁ Understand the language that is used in the procedure. Procedures use language such as, “weigh 10 grams of substance to the nearest tenth of a gram.” This means that a sample is desired to be approximately, but not necessarily exactly, 10 grams, and when recording the mass, make sure to record significant digits to the appropriate place based on the balance used; in this lab, 0.1 grams. In this example, a sample between 9.5 and 10.5 grams would be sufficient as long as you record the mass to the precision of the balance. If a procedure says to “weigh exactly 10 grams,” then care should be given to get as close to 10 grams as the balance will allow while still recording significant figures based on the precision of the balance.
⦁ It is important that laboratory balances are properly calibrated. Because balances are used frequently and by many people, small errors in the absolute masses of objects can be slowly introduced over time. To minimize such errors, masses in the laboratory should be determined using the “difference method.” Mass determination by the difference method is done in the following way:
⦁ Weigh the empty container to be used on the balance. ()
⦁ Add the sample to be measured to the empty container. ()
⦁ Subtract the Mass of the Container from the Mass of the Container and Sample to get the difference, which will be the mass of the sample. ()

⦁ Record mass measurements that reflect the precision of the balance—that is, the correct number of significant figures (see Laboratory Data, Part B). Different electronic balances, having varying degrees of sensitivity, are available for use in the laboratory. It is important to know (by reading the Experimental Procedure) the precision required to make a mass measurement and then to select the appropriate balance. It may save you time during the data analysis. These balances are shown in Figures T.6a through T.6c.

Required Materials
⦁ A balance with at worst 0.1 g precision. This can be a laboratory or kitchen balance.
⦁ Containers to be used for weighing samples. You can use small bowls or plates, measuring cups, ¼ sheet of paper (for solid object), etc.
⦁ Measuring cup or beaker for measuring and pouring the granular solid and water.
⦁ A small solid object small enough to fit into a container placed on the balance.
⦁ At least 50 g of a granular solid. This can be any type of granular solid; for example, sand, salt, sugar, baking soda, etc.
⦁ 1 quart of water.
Safety Precautions
If you choose objects that are breakable, be careful not to drop them.
Be careful with the solid object, granular solid, and water; keep them away from your eyes.
Note: all of the materials used should be safe to use in a typical kitchen.
Procedure
In quantitative analytical work always record the balance readings directly into your data and observations.
Place the balance on a flat, hard, level surface. Turn on the balance and push the tare or zero button.
A. Determining the mass of the solid object
⦁ Choose a Solid Object whose mass you will measure and describe the Solid Object in the appropriate location on the Data sheet.
⦁ Choose a container to use for determining the mass of the Solid Object; this will be Container A. Describe the type of container used in the appropriate location on the Data sheet.
⦁ Place Container A on the balance, then record the Mass of Container A on the Data sheet to the nearest 0.1 g.
⦁ Place the Solid Object into Container A, then record the Mass of Container A and the Solid Object to the nearest 0.1 g.
⦁ Determine the Mass of the Solid Object by subtracting the Mass of Container A from the Mass of Container A and the Solid Object. Record the value on the Data sheet.
⦁ Remove Container A and the Solid Object from the balance, then repeat Steps A3-A5 two more times for a total of 3 trials.
⦁ After performing 3 trials, calculate the average value.
⦁ Now place Container A on the balance and push the tare or zero button.
⦁ Place the Solid Object into Container A, take a picture to upload with your report, then record the mass to the nearest 0.1 g to determine Mass of the Solid Object without using the difference method. Record this mass on your Data sheet.
B. Determining the Mass of a Granular Solid
⦁ Choose a Granular Solid whose mass you will measure and describe the Granular Solid in the appropriate location on the Data sheet.
⦁ Choose a container to use for determining the mass of the Granular Solid; this will be Container B. Describe the type of container used in the appropriate location on the Data sheet.
⦁ Place Container B on the balance, then record the Mass of Container B on the Data sheet to the nearest 0.1 g.
⦁ Measure out either ¼ cup (2 fluid ounces) or 50 milliliters of Granular Solid using the measuring cup and place it into Container B, then record the Mass of Container B and the Granular Solid to the nearest 0.1 g. Indicate whether you used ¼ cup or 50 mL and take a picture to upload with your report.
⦁ Determine the Mass of the Granular Solid by subtracting the Mass of Container B from the Mass of Container B and the Granular Solid. Record the value on the Data sheet.
⦁ Remove Container A and the Granular Solid from the balance, then repeat Steps B3-B5 two more times for a total of 3 trials.
⦁ After performing 3 trials, calculate the average value.
C. Determining the Mass of a Liquid
⦁ Choose a container to use for determining the mass of water; this will be Container C. Describe the type of container used in the appropriate location on the Data sheet.
⦁ Place Container C on the balance, then record the Mass of Container C on the Data sheet to the nearest 0.1 g.
⦁ Measure out either 1 cup (8 fluid ounces) or 250 milliliters of water using the measuring cup and place the water into Container B, then record the Mass of Container C and Water to the nearest 0.1 g. Take a picture to upload with your report.
⦁ Determine the Mass of Water by subtracting the Mass of Container C from the Mass of Container C and Water. Record the value on the Data sheet.
⦁ Remove Container A and the Water from the balance and pour out the water. Dry Container C, then repeat Steps C3-C5 two more times for a total of 3 trials.
⦁ After performing 3 trials, calculate the average value.

 

 

 

 

 

 

 

Pre-Laboratory Questions
⦁ Describe how to use the “difference method” when performing mass determinations. Why are masses in the chemistry laboratory usually determined by the difference method (using a beaker to contain the object to be weighted, rather than just placing the object directly on the pan of the balance)?

 

 

 

 

 

 

⦁ Why should the mass of an object never be determined while the object’s temperature is above room temperature?

 

 

 

 

⦁ If you weigh a hot object on the balance, the reading will be higher, the same, or lower than if it had been weighed after it cools to room temperature?

 

 

 

 

 

 

⦁ Why should liquids never be poured on or near the balance?

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Results and Observations
A. Determining the mass of the solid object
Description of the object:
Description of Container A:
Trial 1 Trial 2 Trial 3
Mass of Container A _____________ _____________ _____________
Mass of Container A and the Solid Object _____________ _____________ _____________
Mass of the Solid Object _____________ _____________ _____________
Average Mass of the Solid Object _____________
Mass of the Solid Object (w/o difference method) _____________
B. Determining the Mass of a Granular Solid
Description of the Granular Solid:
Description of Container B:
Trial 1 Trial 2 Trial 3
Mass of Container B _____________ _____________ _____________
Mass of Container B and the Granular Solid _____________ _____________ _____________
Mass of the Granular Solid _____________ _____________ _____________
Average Mass of the Granular Solid _____________
C. Determining the Mass of a Liquid
Trial 1 Trial 2 Trial 3
Mass of Container C _____________ _____________ _____________
Mass of Container C and the Water _____________ _____________ _____________
Mass of the Water _____________ _____________ _____________
Average Mass of the Water _____________
* Upload one picture for each part of the lab (A, B, and C) with your report sheet
Post-Laboratory Questions
After determining the mass of the Solid Object using the difference method, you tared the balance with the Container A on it, then placed the Solid Object into Container A to determine its mass. Did the resulting mass determination agree with that determined using the difference method? Explain why your results do or do not make sense.

 

 

 

Why is it important always to use the same balance during the course of an experiment? Explain using examples from your own data.