Measuring Mass using a Centigram and an Electronic Balance Introduction: The object of this experiment is to compare the results obtained using two different equipments and methods of mass measurement in Model ChemLab. The mass measurement instruments that are to be compared in this experiment are:
The two techniques to be used in mass measurement are:
Weighing by difference
The experiment is divided into four parts:
Measuring mass directly using a centigram balance
1) Select centigram balance from equipment selection in the toolbar.
2) Right click on the balance and from chemicals select copper slug changing the quantity from 0 to 1.
3) Record the reading obtained in grams
Measuring mass by weight difference method using a centigram balance
1) Add a centigram balance to the workspace
2) Add a 100mL from equipment selection using the same method and record its mass
3) Next add the copper slug from the previous measurement to the beaker. To do this, drag the beaker by left clicking on it below the first balance which contains the copper slug. Now right click on the first balance and select pour/decant. The copper slug will drop into the beaker. To measure the total mass of the beaker and the copper slug drag them to the second balance. Record the reading
4) Obtain the mass of the copper slug by subtracting the mass of the beaker from the total mass of the beaker and the slug.
Measuring mass directly using an Electronic Balance
1) Left click on the balance with slug, select transfer to new container and click on electronic balance.
2) Record the reading
Measuring by Weight Difference method using an Electronic Balance
1) Left click on the balance with the beaker and slug from Part 1 of the experiment and use the method given above to transfer it to an Electronic Balance.
2) Record the reading
3) Remove the beaker from the balance and empty it by right clicking and selecting Empty.
4) Place the beaker on the balance again and record the reading.
5) Subtract the reading obtained in step 4 from the reading obtained in step 2 to get the mass of the copper slug.
Calculating the Average Mass of a copper slug using an Electronic Balance
1) Obtain an Electronic Balance using the method described earlier
2) Add a copper slug using the method described earlier and record the mass
3) Add a second copper slug and record the mass
4) Add a third copper slug and record the mass
5) Add a fourth copper slug and record the mass
6) Add a fifth copper slug and record the mass
7) Divide by 5 the reading obtained in step 6 to obtain the average mass of a copper slug
Determining the Mass of Unknown Weights
1) Obtain a Centigram Balance
2) Add beaker and record the reading
3) To the beaker add Unknown Weight 1 and record the total mass of both
4) Subtract the reading obtained in step 2 from the reading obtained in step 3 to get the mass of Unknown Weight 1
5) Repeat the same steps for Unknown Weight 2
Observations and Results
Weighing by Difference
Mass of 100ml beaker
Mass of beaker+slug
Mass of slug:
(mass of beaker+slug)-(mass of beaker)
Average Mass of a Copper Slug with an Electronic Balance:
Mass of 1 copper slug/g
Mass of 2 copper slugs/g
Mass of 3 copper slugs/g
Mass of 4 copper slugs/g
Mass of 5 copper slugs/g
Average mass of a copper slug/g
Determining the Mass of Unknown Weights with a Centigram
Mass of 100ml beaker
Mass of beaker+weight/g
Mass of weight:
(mass of beaker+weight)-(mass of beaker)
Unknown Weight 1
Unknown Weight 2
The Centigram Balance, which gives a reading of 2. 3 on weighing the copper slug directly, has a sensitivity of 0. 1 grams, whereas the Electronic Balance which gives a reading of 2. 999 for the same slug has a much higher sensitivity of 0. 001g. It is important to use the same slug for both instruments to make sure that any disparity in the readings is due to the instrument being used and not because of a difference in the mass of the copper slug being measured. The Electronic Balance has a higher accuracy. This is further proved by the value obtained for average mass of a copper slug using an Electronic Balance. This value, 2. 980, is very close to the reading 2. 989 which was obtained when a copper slug was measured on the Electronic Balance using weight difference method. The Electronic Balance, due to its high sensitivity and accuracy is more reliable for measurement of small masses. The Centigram on the other hand has a much lower accuracy and sensitivity and is thus unsuitable for small masses.
Weighing by difference is different from weighing directly because it involves taking two readings and then subtracting the two to get a value for the mass that has to be measured. Weighing by difference gives more accurate results because when the readings are subtracted then any zero error due to wrong calibration of the instrument will be removed from the final reading. In case of weighing a mass directly if the instrument contains a zero error it will show up in the reading obtained which will then be inaccurate.
Conclusion: This experiment establishes that the use of an Electronic Balance as opposed to a Centigram Balance and the use of weighing by difference technique as opposed to weighing directly produce more accurate results.