Mass Determination

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Table of Contents:

Introduction
Choice of Balance to Be Used

Introduction

Chemistry became a quantitative science only after the importance of mass determinations was understood by chemists and sensitive analytical balances were developed. Mass determinations in this course are done using electronic top-loader balances (not scales). The balances are on the lab bench beneath the windows. As described below, there are three different types of balances, reflecting trade-offs between cost, speed and sensitivity. The correct choice of the balance will help you optimize your time, accuracy and precision. All of these balances are delicate, expensive instruments. Always treat a chemical balance with utmost care.

Sample masses reported in your experimental work will actually be the difference between two balance readings, an initial and a final reading.
If the weighing involves sample addition, an empty container is initially weighed. Sample is then added to the container and a final mass recorded.
Many balances have a tare capacity which allows the balance reading to be adjusted to zero with an empty container on the pan.
After the container has been tared, it is removed to a clean surface, sample is added, and the container plus sample is returned to the balance pan. The balance reading is then the actual sample mass.

Two rules will be strictly enforced:

In quantitative analytical work always record the balance readings directly into your notebook.
This means the laboratory notebook must be nearby when using a balance. Do not try to "remember" a number for any length of time.
No chemical should ever be added to or removed from a container while it is on a balance pan.
Chemical spills onto a balance pan can affect measurements for an entire class.

Choice of Balance to Be Used

Select the balance to be used according to the sensitivity that you need. Many of the masses in this course can be determined to sufficient precision using a 0.1 or 0.01 gram top-loader. Always use the 0.1 or 0.01 gram balance if you want the mass to the nearest gram or the nearest 0.1 g.
If the mass must be determined to the nearest mg (or 0.001 g), use the electronic top-loader that is located in your lab. If the mass must be determined to the nearest 0.1 mg (or 0.0001 g), use an analytical balance in your lab.

Use of 0.01 g and the 0.1 mg Electronic Top-Loader Balances

Before weighing an object on the electronic top-loader, make sure the pan is clean (report to your assistant or instructor if you find chemicals spilled on the pan)
To turn on the balance press down lightly on the control bar on the front. The balance should be left on throughout a regular lab day in which it is being used frequently.
If you wish to weigh an object directly, press down gently on the bar again to get a reading of zero (be patient, sometimes you will have to wait until the balance does not feel vibrations). Place the object on the pan, and record the reading in your lab notebook.
If you wish to let the tare of the balance do your subtracting for you, place the empty container on the balance pan first, then gently press down on the bar and wait for the balance to read zero. Carefully move the empty container to your research book or other clean surface where sample may be added to the container. Be sure that no one touches the control of the balance while you are doing this. Replace the container with sample on the balance pan, and record the final balance reading in your notebook. In this case, the balance readout is the sample mass. Never touch the balance pan with your fingers.

Use of the Milligram Electronic Top-Loader Balances

The operation of these balances is essentially the same as the 0.1 g balance except that the more sensitive balances are disturbed by air currents, drafts, and slight vibrations.
They are more expensive than the least sensitive balances and take more time to give a stable reading.
To avoid draft interference, always use the draft shield and close the shield windows when a balance reading is made and when the balance is being tared. To minimize vibrational interference, do not lean on the balance support table. Close the windows on the balance when you finish using it.
Care must be taken in handling sample containers when mass determinations sensitive to 1 - 0.1 mg are made. Your hands must be clean and free from perspiration; careful handling with dry fingers when determining the mass to the nearest mg is acceptable.
It is essential that all objects be allowed to reach room temperature before they are weighed. If the temperature of the object differs from that of the balance, convection currents are established in the balance that result in erratic balance readings. This is one of the most common sources of difficulty in attaining "constant mass" in sequential weighings.

Use of the 0.1 mg Electronic Top-Loader Balances: the Analytical Balances

The operation of these balances is the same as the mg balances except that they are ten times as sensitive. Their increased sensitivity makes them more sensitive to vibrations; and therefore, slower in giving a readout. Be patient with them and treat them very gently.
Special care must be taken in handling sample containers when mass determinations sensitive to 0.1 mg are made. Careful handling with dry fingers when determining the mass to the nearest mg is acceptable. When determining mass to the nearest 0.1 mg, direct finger-glass contact should be avoided. Small beakers and weigh bottles can be conveniently handled by folding a piece of paper to form a 2 or 3 cm wide strip of paper which can be shaped into a loop and pinched around the container without making glass-finger contact. Crucible tongs can also be used to handle containers between weighings.
One added adjustment that you must check on the analytical balance is that it is level. Check the level bubble to insure that the balance is level before beginning to use it.