Measuring Mass Using a Container

1. Many solids and all liquids require a container when a triple beam balance is used to find mass. When a container is used, its mass has to be subtracted from the mass of the container and the substance so that the mass of the substance alone can be found.

2. There are two methods using a weigh dish or beaker when measuring mass.

Measuring a Specific Mass of Substance:

3. Determine the mass of the container (weigh dish or beaker) by placing the container on the platform of the triple beam balance. Move the poises until the indicator mark aligns (matches up) with the center graduation mark.

4. Record the mass of the container.

5. Move the poises so that they equal the mass of the container plus the mass of the substance you wish to obtain.

For example, suppose you want to obtain 10 g of a sample of baking soda. You find that the mass of the weighing dish equals 3.8 g. To obtain 10 g of baking soda, the poises should be moved to 13.8 g.

Mass of weighing dish + Mass of substance to be obtained = Total mass shown by poises

3.8 g + 10 g = 13.8 g baking soda

6. Add the substance(baking soda) to the container until the indicator mark aligns with the center graduation. When the indicator mark is aligned, you have obtained the desired amount of the substance.

7. Some substances are solids that are not easily divided or separated and the indicator may not align properly with the center graduation. When this happens, move the poises until the two marks align. Record the final mass of the container plus the substance and subtract the mass of the weighing dish to find the final mass of the substance.

For example, you may want 10 g of pinto beans but cannot cut the beans so that the triple beam balance reads 13.9 g. However, the indicator will align at 14.2 g. In this case, you would subtract the mass of the weighing dish from the mass of the weigh dish plus the beans, and have a sample with a mass of 10.3 g:

Mass of weighing dish plus beans – Mass of weighing dish = Mass of beans

14.2 g – 3.9 g = 10.3 g beans

Determining the Mass of a Sample:

8. Determine the mass of the container.

9. Record the mass.

10. Add the sample to the container.

11. Adjust the poises so that the indicator line and center graduation align.

12. Calculate the total mass shown on the three poises.

13. Subtract the mass of the container from the mass of the container plus the sample to find the mass of the sample.

For example, suppose you want to find the mass of a sample of baking soda. The weigh dish has a mass of 3.9 g. The total shown on the poises equals 35.0 g. Subtracting 3.9 from 35.0 gives 31.1 g. The sample has a mass of 31.1 g.

Mass of weighing dish plus baking soda – Mass of weighing dish alone = Mass of baking soda

35.0 g – 3.9 g = 31.1 g baking soda