exp 1 - the analytical balance.pdf

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CHM130L. Analytical Chemistry Laboratory -1

EXPERIMENT 1

USE OF THE ANALYTICAL BALANCE

OBJECTIVES

Upon completion of the experiment, the student should be able to:

1. define the principles and proper techniques involved in using the analytical

balance;

2. use the analytical balance properly;

3. identify the factors affecting accuracy and precision of measurements;

4. use statistics in evaluating data gathered; and

5.

use spreadsheets in analyzing results.

LIST OF CHEMICALS/MATERIALS

Solid NaCl Distilled water

1-peso coins

LIST OF APPARATUS

50 mL beaker Medicine dropper

Wash bottle Crucible and cover

Analytical balance

Crucible tongs

1. Wear laboratory gown or apron during the entire laboratory period and safety

goggles when doing the experiment.

2. Dispense the chemicals in properly labeled containers.

3. Read again the laboratory rules on safety before proceeding to the experiment.

DISCUSSION

Mass is a fundamental property of matter that is commonly determined in

analytical measurements. Mass determination with the use of a balance serves many

purposes in chemical analysis, starting from sample and reagent preparation until

determination of amount of products formed from a chemical reaction. The use of the

balance offers wide advantages due to speed, accuracy, reproducibility, and inexpensive

method of analysis.

SAFETY PRECAUTIONS



Experiment 1. The Use of the Analytical Balance

CHM130L. Analytical Chemistry Laboratory Page 1-2

When objects are placed in a balance, it is the net force of gravity plus other

forces acting on the object (weight) that is being determined, not the amount of matter

of 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 (Hage and Carr 2011). The unknown object and the reference object areplaced on opposite sides of the balance, and therefore will experience the same forces

from the surrounding medium and the same local gravitational field (Figure 1). The

difference in force between the two sides should be directly related to their difference

in mass.

There are many types of balance used for chemical analyses. The type of the

balance to be used for a particular mass measurement considers capacity (maximumload), readability (smallest division that the balance can display), and resolution

(mathematically determined by dividing capacity with readability) (Hage and Carr 2011).

In analytical measurements, most weights are in the order of a few grams to a few

milligrams or less and values reported usually contain three or four significant figures.

The balance to be used must not only give accurate results, but it should also be

sensitive (low readability values) and for this case, the balance most useful is the

analytical balance (Christian 2004). The analytical balance can usually measure at least

0.1 milligram and contains a weighing compartment that is enclosed (Figure 2) (Hage

and Carr 2011).

PRE LAB QUESTIONS:

1. Using the concept of the forces acting on an object in the determination of

weight, derive the equation used to obtain the true mass of an unknown object.

2. Differentiate these types of weighing: 1) direct weighing, 2) weighing by addition

3) weighing by difference.

Figure 1. Forces acting on a reference object

and an unknown objectFigure 2. The analytical balance





3. Why must the following precautions be observed when using an analytical

balance?

a. Balance door must be closed while weighing

b. Objects must not be weighed hot

c. Bubble must be at the center

d.

Do not lean on the table carrying the balance

PROCEDURE

I. ANALYTICAL WEIGHING

1. Determine the mass of a 50-mL beaker by using a crucible tong in placing the

object on the balance pan. Record the weight. Using your bare hands, remove

the beaker from the balance and then reweigh. (Postlab Q1)

2. The mass of a solid sample transferred to a container can be calculated using

two ways. The succeeding procedure allows you to differentiate one from the

other and provides you insight on when best to use each method.

2.1. Obtain a crucible and cover from the Chemistry preparation room. Weigh

the crucible cover and record its weight.

2.2. Weigh the two objects (crucible and cover) combined and record the

weight.

2.3. Remove the crucible cover and obtain the weight of the crucible.

2.4. Get the difference in the last two weighings to represent the weight of

the crucible cover. (Postlab Q2)

2.5. Obtain a weighing bottle containing NaCl sample, a wash bottle

containing distilled water, and a dry 50-mL beaker. Determine the mass

of the bottle with NaCl, the mass of the wash bottle, and the mass of the

dry beaker, separately.

2.6. Using a tissue paper to hold the NaCl bottle, transfer most of the NaCl to

the beaker. Be careful not to lose any sample. There may still be a little

left on the bottle. Determine the mass of the weighing bottle again.

2.7. Determine the mass of the beaker containing NaCl.

2.8. While the beaker is still on the balance, use the wash bottle to transfer

approximately 10-15 mL of water to the beaker. Be careful not to lose

any. Reweigh the beaker containing NaCl and water.

2.9. Reweigh the wash bottle.

2.10. Use your data from 2.5 to 2.8 to compute for the mass of the salt.

Compare your results in weighing by addition and weighing by

difference.





2.11. Use your data from 2.5 to 2.9 to compute for the mass of the water

transferred to the beaker. Compare your results in weighing by addition

and weighing by difference.

II. STATISTICAL EVALUATION OF MEASUREMENTS

1. THE DIFFERENCE IN ONE “DROP”

1.1. Fill a 50-mL beaker (beaker A) with 20 mL distilled water. Dry the sides of

the beaker prior to weighing. Weigh the beaker with water and a

medicine dropper.

1.2. Using another clean beaker (beaker B), transfer exactly one drop of water

from beaker A to beaker B. Place the dropper back in beaker A and

reweigh this beaker and the added water. Do this procedure 10 times.

Calculate the average drop mass, the average deviation and the range.1.3. Deliver ten drops in rapid succession into beaker B and reweigh beaker A

with the medicine dropper. Calculate the average drop mass, the average

deviation and the range. (Postlab Q3)

2. THE DIFFERENCE IN ONE PESO COINS

2.12. Obtain ten (10) 1-peso coins. We would assume that these 10 coins are

representative samples of a coin produced by one particular machine at

the same time.

2.13.

Weigh each coin and record the mass.

2.14. Using the weight of the 1-peso coin determined earlier, calculate the

following a) Mean b) Average deviation c) Range d) Standard deviation e)

Coefficient of variation f) Confidence limits of the mean at 90%

probability level. Use the Q-test to reject any questionable result.

(Postlab Q4)

REFERENCES

Christian, Gary D. 2004. Analytical chemistry (6th ed.). New Jersey: John Wiley and

Sons Inc.Hage, David S. and James D. Carr. 2011. Analytical chemistry and quantitative

analysis. New Jersey: Pearson Prentice Hall.

Madamba, Lilia S.P. 1995. Chemistry 32 Laboratory Instruction Manual (3rd

rev). Los

Baños: Analytical and Environmental Chemistry Division, Institute of

Chemistry, University of the Philippines Los Baños.





Name: Date Performed:

Course/Section: Date Submitted:

Group No: Instructor:

REPORT SHEET

Experiment No: 1

Use of the Analytical Balance

INTRODUCTION:





RESULTS AND INTERPRETATIONS

II. ANALYTICAL WEIGHING

Table 1. Effect of fingerprints in weighingCondition Mass (g)

Mass of beaker using crucible tongs

Mass of beaker handled with bare hands

Post lab Q1. 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.

Table 2. Comparison of methods of weighing

Conditions Mass / Volume

2.1. Mass crucible cover, g2.2. Mass crucible and cover, g

2.3. Mass crucible, g

2.4. Mass crucible and cover – mass crucible only, g

2.5.a. Mass of bottle with NaCl, g

2.5.b. Mass of wash bottle, g

2.5.c. Mass of dry beaker, g

2.6. Mass of bottle – NaCl, g

2.7. Mass of beaker + NaCl, g

2.8. Mass beaker + NaCl + water, g2.9. Mass wash bottle – water, g

2.10. Mass salt (by addition) (2.7. – 2.5.c.), g

Mass salt (by difference) (2.5.a. – 2.6), g

2.11. Mass water (by addition) (2.8. – 2.7.), g

Mass water (by difference) (2.5.b. – 2.9.), g





Post lab Q2. 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.

II. STATISTICAL EVALUATION OF MEASUREMENTS

Table 3.a. Determination of mass of individual drop

Conditions Mass, g

Mass beaker + water + medicine dropper

Mass beaker + water + medicine dropper – 1st

drop

Mass beaker + water + medicine dropper – 2nd

drop

Mass beaker + water + medicine dropper – 3rd

drop

Mass beaker + water + medicine dropper – 4th

drop


drop


drop

Mass beaker + water + medicine dropper –

7th drop


drop


drop


drop

Table 3.b. Mass of individual drop

Drop Mass, g

1st

drop

2nd

drop

3rd

drop

4th

drop

5th

drop





6th

drop

7th

drop

8th

drop

9th

drop

10th

drop

Total mass of 10 drops

Average mass of an individual drop

Average deviation

Range

Table 3.c. Mass of 10 drops in succession

Conditions Mass, g

Mass beaker A + water + medicine dropper

Mass beaker A + water + medicine dropper – 10 drops

Total mass of 10 drops

Average mass of an individual drop

Post lab Q3. Is there a difference in the volume of each drop obtained by the two

different methods? Explain the absence/presence of any difference.

Table 4. Mass of individual coin and statistical analysis

Coin Mass, g Coin (in increasing mass)* Mass, g

1st

coin

1

2nd

coin 2

3rd coin 3

4th

coin 4

5th

coin 5

6th

coin 6

7th

coin 7

8th

coin 8





9th

coin 9

10th

coin 10

Mean

Average deviation

Range

Standard deviation

Coefficient of variation

Confidence limits of the

mean (90% level)

Q-test

*arrange the mass of coins from lightest to heaviest

Post lab Q4. Analyze the statistical data obtained regarding accuracy and precision ofthe measurements. Relate your explanation with regards to how the coins are made by

the same machine.

Signature: ______________

Date completed: ______________

exp 1 - the analytical balance.pdf

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