Experiment 1 - Use of the Analytical Balance

JUNE 2, 2013

USE OF THE ANALYTICAL BALANCE EXPERIMENT # 1

ADORNA JR., JOEMER A. PITAGAN, PAULA JESSIKA C.

MALAYAN COLLEGES LAGUNA

EXPERIMENT # 1

USE OF THE ANALYTICAL BALANCE

I.

II.

OBJECTIVES 

To discuss the principle of the balance and functions of an analytical balance;



To use the analytical balance properly;



To identify the factors affecting accuracy and precision of measurements; and



To use statistics in evaluating measurements.

LABORATORY APPARATUS / CHEMICALS Equipment/Accessories

III.

Quantity

50-mL beaker

2

Analytical balance

1

Wash bottle

1

Pasteur pipette

1

Crucible and cover

1

Crucible tongs

1

Solid NaCl

1

DISCUSSION OF FUNDAMENTALS Measurements are fundamental in our everyday lives. From how beams and trusses’ lengths

should be, to the amount you add to neutralize a solution, it should be accurate and precise. One of the most important measurements in chemical analyses is mass. As a fundamental property of matter, mass should also be measured accurately and precisely by using a balance. When objects are placed in a balance, it is the net force of gravity plus other forces acting on the object (mass). The conversion from weight to mass is done by making use of a reference object with a known mass and comparing the object of unknown mass to this reference. The unknown object and the reference object are placed on opposite sides of the balance, and therefore will experience the same forces from the surrounding medium and the same local gravitational field. The difference in force between the two sides should be directly related to their difference in mass. There are many types of balances, but the most preferred type of balance in terms of capacity, readability and resolution is the analytical balance. This is very much needed in the field of analytical chemistry, for measurements are vital in its experiments.

Experiment 1: Use of the Analytical Balance

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Figure 1. Analytical balance

An analytical balance is a precise and accurate instruments used to measure masses. It can measure masses to within 0.0001 g. Due to its sensitivity, it requires a location on a solid bench or platform that is free of vibrations. Some modern balances have built-in calibration masses to maintain accuracy. Older balances should be calibrated periodically with a standard mass.


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USE OF THE ANALYTICAL BALANCE IV.

METHODOLOGY The mass of a 50-mL beaker was

The masses were then computed. The

determined.

results of weighing by addition and difference were compared.

After obtaining a crucible and cover, the weigh of the cover was recorded.

Approximately 10 mL of water was transferred to the beaker, then it was rewei hed.

The crucible and cover ’s weigh were recorded. The mass of the beaker with NaCl was recorded.

After removing the cover, the weight of the crucible was determined. Transferring the NaCl to a beaker carefully, the mass of the weighing bottle was taken again. The difference of the last two weighings were taken to represent the weight of the cover. The masses of the following were determined: a weighing bottle containing NaCl sample, a wash bottle containing distilled water, and a dry 50-mL beaker.

Figure 1.2. Analytical weighing experiment

The experiment consisted of different parts showing how different weighing methods are made. Such methods were weighing by addition and weighing by difference. In the experiment, the weighing by difference was shown in the determination of the crucible’s weight without directly putting it in the balance. The weighing by difference was shown in the det ermination of the weight of the NaCl sample.


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EXPERIMENT # 1

USE OF THE ANALYTICAL BALANCE After filling a 50-mL beaker with 20-mL

Ten one-peso coins were collected.

distilled water, the beaker with the water and medicine dropper was weighed.

Each coin was weighed, masses recorded. Using a new beaker (B), exactly one drop from beaker A was transferred. The new beaker was reweighed with the added water (10 times). Average drop mass,

Using the weight, the following were

average deviation and range were taken.

determined: mean, average deviation, range, standard deviation, coefficient of variation, confidence limits of the mean at 90% probability level. Q-test was used

Ten drops were delivered in rapid succession into beaker B and beaker A was reweighed. Calculated: average drop

to reject questionable result. Figure 1.4. The difference in one peso coins

mass, average deviation, range.

Figure 1.3. The difference in one “drop”

The next procedures consisted of statistical evaluation of the given measurements of the analytical balance. The results of our experiment gave a relatively low percentage for the relative standard deviation, thus meaning that the methods performed were very precise. This also means that the data given by the analytical balance were reliable and accurate.


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V.

DESCRIPTION OF THE APPARATUS / SET –UP

VI.

DATA SHEET Object

50-mL beaker (A) Crucible cover

Mass (g) 30.159 Direct:10.674 / By difference: 10.675

Crucible and cover

39.976

Crucible

29.301

Weighing bottle with NaCl

3.544


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Wash bottle with distilled water and NaCl

Initial: 156.661 / Final: 146.896

50-mL beaker (B)

30.158

Weighing bottle

2.527

Beaker with NaCl

31.612

Beaker with NaCl and water

41.056

Table 1.1.

Mass of Water transferred to beaker Weighing by addition

9.444g

Weighing by difference

9.765g

Table 1.2.

Mass of Salt Weighing by addition

1.452g

Weighing by difference

1.017g

Table 1.3

X (Mass of beaker + drop) (g)

Xn-Xn-1 (g)

30.199 30.235

0.036

30.269

0.034

30.304

0.035

30.338

0.034

30.375

0.037

30.409

0.037

30.446

0.037

30.484

0.035

30.518

0.034 One drop: 0.0359 / Rapid: 0.0394

Average drop mass (ADM) Range

3.0 x 10

-3

Table 1.4. 2

Coin

Mass (x)

-  

-  

1

5.455

0.0041

1.94x10-3

2

5.318

-0.929

8.63x10-3


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3

5.535

0.1241

1.54x10-2

4

5.436

0.0251

6.30x10-4

5

5.459

0.0481

2.31x10-3

6

5.423

0.0121

1.46x10-4

7

5.368

-0.0429

1.84x10-3

8

5.304

-0.1069

1.14x10-2

9

5.387

-0.0239

5.71x10-4

10

5.424

0.0131

1.72x10-4

Total

54.109

0.043039

Table 1.5

 =5.4109

R=0.155 s=0.06918

VII.

SAMPLE CALCULATIONS Mass of water: By difference: 156.661-146.896 = 9.765g By addition : 41.056-31.056

= 9.444g

Mass of NaCl By difference: 3.544-2.527

=1.017g

By addition : 31.612-30.158

=1.452g

 = 0.0359g      =  = 0.0394g

ADMone drop = ADMrapid

  =

 

= 5.4109g

 = 0.06918 √ 

s=

RSD =

 x 100% = 1.28% 


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VIII.

RESULTS AND DISCUSSIONS This experiment is all about the analytical balance, its capabilities, the accuracy and precision of its results, and how it is used. The analytical balance is a very important equipment in the laboratory because mass measurements are one of the basic information supplied and required in an experiment. As to prove the accuracy of its results, we tested it with different trials and supported our results with statistical analysis. First off, we compared the mass results of two samples, without directly weighing it, using two different methods: weighing by addition and weighing by difference. The results were close to each other considering the many steps they had gone through. As for precision, we measured 10 drops of water, which is of no accurate and definite mass. The standard deviation computed, 0.0691, shows that the masses we have recorded are not dispersed and are actually close to each other. Considering that a drop of water has a small mass and require many significant figures in displaying, the result is fairly precise. Errors accounted for were most probably caused by human intervention, including leaning on the bench where the balance is placed, not closing the doors, and touching the samples with bare hands. This was also done with 10 individual one-peso coins.

IX.

SUMMARY AND CONCLUSIONS This experiment in overall proved the accuracy in the calibration of one of the most notable/ important instruments in analytical chemistry, the analytical balance. The analytical balance, unlike all other balances, gives more precision in its measurements by having three to four significant figures, sometimes even ten thousandths in decimal places. The balance is placed on a laboratory setting, yet is sensitive to air pressure and resistance. As observed throughout the whole experiment, this is why the following precautions that were stated in the prelaboratory guide must be followed for getting bette r results. In knowing how precise our measurements would be, we used statistics primarily with our data sets, where we proved on how deviated our data are. A specific example would be getting the


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USE OF THE ANALYTICAL BALANCE RSD, or the coefficient of variation of the data set. We got a low percentage, which generally means that we got a very high precision rate. Overall, mass measurements using an analytical balance should be: significant in displaying results, can handle a considerable amount of weight (a good capacity) and by following certain precautions to ensure accuracy and precision in measurements, especially in mass.

X.

POST LAB QUESTIONS 1. Based on the above observations, what can be concluded about the effect of fingerprints on weighing objects? Suggest other means of weighing without making use of bare hands aside from using crucible tongs. The mass of the beaker would be bigger when handled by bare hands, as opposed to holding it with crucible tongs. This is possibly due to the oil that c ertain glands in the skin secrete, e ven in the palm of our hands. We can m ake use of weighing the beaker without directly touching it, by touching it using gloves, ironically. This method will not transfer the oils in your palms, making your measurements more accurate. 2. Get the difference of the crucible masses obtained in 2.1 and 2.4. Does the mass obtained by difference agree within 0.5 mg with the mass obtained by direct weighing? Account for any difference greater than 0.5 mg. |10.674g-10.675g|=0.001g. This is equivalent to 1 mg, which did not agree to a difference of 0.5 mg, since the analytical balance that we used only displayed 3 decimal places, not the supposed way around. 3. Is there a difference in the volume of each drop obtained by the two different methods? Explain the absence/presence of any difference. There is a significant difference present in the volume obtained by the two different methods. This is mainly due to surface tension, and air resistance. Since the ‘per drop’ method lets the

water stop for every drop transferre d, surface tension applies on it significantly, not unlike the ‘rapid succession’ method. This difference ultimately results to a greater average drop mass of

the latter method, since there are no recurring retarding forces present.


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USE OF THE ANALYTICAL BALANCE 4. Analyze the statistical data obtained regarding accuracy and prec ision of the measurements. Relate your explanation with regards to how t he coins are made by the same machine. Since we got a low range and standard deviation, we can say that our accuracy and precision for this procedure is high. With this, we can say that the coins that are manufactured in the mint were mass produced, but the mass w as estimated to such accuracy. This is mainly because to oppose counterfeiters.

XI.

REFERENCES Christian, Gary D. 2004. Analytical chemistry (6th ed.). John Wiley and Sons Inc. Hage, David S. and James D. Carr. 2011. Analytical chemistry and quantitative analysis. New Jersey: Pearson Prentice Hall. Skoog, Douglas et. al. 2004. F undamentals of Analytical Chemistry (8th ed.). Singapore: Thomson Learning.


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Experiment 1 - Use of the Analytical Balance

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