massing

Measuring Mass

The measurement of mass in the laboratory is performed using balances. The type of balance that we use here at CAC are triple-beam balances. Here is a basic review of techniques to properly use this important laboratory instrument.

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.

To actually measure the mass on the triple-beam balance:

1. After placing object on balance pan, move the heaviest counter-balance (100's) over 1 complete notch at a time until the ...pointer goes below the zero mark. then back off 1 notch (100 grams) so the pointer is above the zero mark
2. Repeat the same process with the 10 gram counterbalance
3. Contimnue with the smallest counterbalance (0.10 gram) but this time stop when the pointer points directly at the zero mark.
4. Sometimes using a pencil to bump the smallest rider helps to move the counterbalance minute amounts.
5 Read the mass directly from the last mark passed on the balance beam, then estimate the last digit by the distancethe . ...counterbalance pointer is between marks. This last figure is considered part of the measurement.

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.

Information on Direct weighing, weighing by difference and Taring from Virtual Chemistry Lab

For more information on laboratory techniques see: http://www.acp.edu/web/genchem/thedisk/labtech/labtech.htm

	The measurement of mass in the laboratory is performed using balances. The type of balance that we use here at CAC are triple-beam balances. Here is a basic review of techniques to properly use this important laboratory instrument.
	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.
	To actually measure the mass on the triple-beam balance: 1. After placing object on balance pan, move the heaviest counter-balance (100's) over 1 complete notch at a time until the ...pointer goes below the zero mark. then back off 1 notch (100 grams) so the pointer is above the zero mark 2. Repeat the same process with the 10 gram counterbalance 3. Contimnue with the smallest counterbalance (0.10 gram) but this time stop when the pointer points directly at the zero mark. 4. Sometimes using a pencil to bump the smallest rider helps to move the counterbalance minute amounts. 5 Read the mass directly from the last mark passed on the balance beam, then estimate the last digit by the distancethe . ...counterbalance pointer is between marks. This last figure is considered part of the measurement.
	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. Information on Direct weighing, weighing by difference and Taring from Virtual Chemistry Lab For more information on laboratory techniques see: http://www.acp.edu/web/genchem/thedisk/labtech/labtech.htm