Measuring Mass

The measurement of mass in the laboratory is performed using balances. While there are a great variety of balance types, they are all expensive and delicate instruments that need to be treated with respect. Most balances in use now are electronic balances. If this is not the case at your school, you will need to consult your instructor to obtain directions on how to use your balances.

Balance Rules

In order to protect the balances and ensure accurate results, a number of rules should be followed:

1.	Always check that the balance is level before using it.
2.	Never weigh directly on the balance pan. Always use a piece of weighing paper to protect it.
3.	Never add chemicals to a container or piece of weighing paper while it is on the balance.
4.	Do not weigh hot or cold objects.
5.	Clean up any spills around the balance immediately.

Direct Weighing

Direct weighing means that an object is placed directly on a balance and the mass read. Weighing directly requires that the balance be carefully zeroed (reads zero with nothing on the balance pan) in order to obtain accurate results.

Weighing by Difference

Weighing by difference requires that two measurements be taken. These measurements can be taken in one of two ways:

(1)

A clean piece of weighing paper is placed on the balance and weighed. This is the first reading. The substance to be weighed out is then placed on the weighing paper and weighed. This is the second reading. The mass of the substance is determined by subtracting the first reading from the second.

(mass of paper + substance) - (mass of paper) = (mass of substance)

(2)

A container holding the substance of interest is placed on the balance and weighed. This is the first reading. The substance to be used is removed from the container and the container weighed again. This is the second reading. The mass of substance taken is determined by subtracting the second reading from the first.

(mass of container + substance) - (mass of container + substance after removal) =
(mass of substance removed)

When weighing by difference an exact zeroing of the balance is not required because any error in initial reading will cancel when subtraction of the readings occurs. However, it is always good practice to zero the balance when you begin to be certain it is working correctly.

Taring

By taring a balance the process of weighing by difference is done automatically. When a balance is tared with an object on the balance pan, the weight of the object will be automatically subtracted from every reading until the balance is re-tared or zeroed. Consider this example. A beaker is placed on the balance pan and the tare button pushed. Zeroes should appear on the display (the mass of the beaker minus the mass of the same beaker should equal zero). If a substance is now added to the beaker, just the mass of the substance is displayed. Taring is a very convenient method of weighing out chemicals, one you will probably use quite a lot.

See some examples.