Lab Manual CHM138 (eks 1,2&5) PDF

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EXPERIMENT 1 BASIC LABORATORY TECHNIQUES INTRODUCTION Chemistry is an experimental science. It depends upon careful observation and the use of good laboratory techniques. Most of the experiments in the chemistry laboratory involve quantitative analytical procedure. It is involve the use of common glassware for example burette, pipette, volumetric flask etc. These glassware are used to measure the volume of solutions at certain temperature. The volume of a liquid change with temperature, to get the accuracy, the apparatus involve to measure the volume of solutions have to be calibrated before used. Mistakes and errors can happen during an experiment. A mistake is a blunder or unintentional action whose consequence is undesirable. Error on the other hand, account for the range of values obtained from successive measurements of the same quantity, even though there was no mistake in any of the measurement. Error may be either systematic or random. A systematic error can happen when an apparatus which is not calibrated is used. The measurement will always be too large or, alternatively, too small. A systematic error will influence the accuracy of a measurement, that is, the agreement between a measured value of a quantity and its true value. A random error will be the evident when a measuring device, even a very accurate one, is used a number times to make the same measurement. Both error can be reduce by using calibrated apparatus and careful when doing experiment. a)

Volumetric flask

A volumetric flask is glassware designed to deliver the standard solution at precise known volume of liquid at given temperature. The actual volume of liquid in the flask can be determined by weighing the empty flask then flask and distilled water. The difference between both readings is equal to the mass of water. The volume of water in the flask can be calculated from Table 1. Volumetric flasks are used to make solutions of known concentration by the dissolution of a known mass of solid or the dilution of a more concentrated solution. Before use, always wash the flask and then pre-rinse with the solvent. Some frequently used volumes in General Chemistry lab are 10.00, 25.00, 50.00, 100.00, and 250.0 mL flasks. At times the zeros to the right of the decimal point are omitted. However, these zeros must always be considered in calculations, as they indicate the accuracy of the volume measurement. (i.e., they are significant figures).

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b)

Pipette

Pipettes are glass vessels that are constructed and calibrated so as to deliver a precisely known volume of liquid at a given temperature. Transfer and Mohr pipette are two types of common pipette usually found in the laboratory. A transfer pipette is calibrated to deliver only one volume, whereas a Mohr pipette is graduated so that it can deliver any volume (usually to nearest tenth of a milliliter) up to maximum volume. Transfer pipettes come in many size, but 5 mL, 10 mL, 20 mL, 25 mL pipette are usually found in general chemistry laboratories. Common volume of Mohr pipette are 1 mL, 5 mL and 10 mL volume. The correct use of a pipette requires considerable manipulatory skill. Step-by step procedures for correct usage are: a) b) c) d)

Rinse the pipette with the solution to be used. Insert the pipette into the solution and suck using pipette filler (suction bulb). The solution is about 1 to 2 cm above the etched line on the pipette. Drain the excess into a waste container until the bottom of the meniscus coincides with etched line. Allow the liquid in the pipette to drain into the flask to be used in the experiment. Touch off the last drop. (Do not blow the remaining liquid from the pipette. The pipette was calibrated to deliver the correct volume with this liquid remaining in it).

The actual volume of solution (pipette volume) can be measured by weighing the solution that has been transferred using that pipette. From the density of the solution, we can calculate the volume of solution (pipette volume). c) Burette A burette (also buret) is a vertical cylindrical piece of laboratory glassware with a volumetric graduation on its full length and a precision tap, or stopcock, on the bottom. It is used to dispense known amounts of a liquid reagent in experiments for which such precision is necessary, such as a titration experiment. Burettes are extremely accurate - a 50 mL burette has a tolerance of 0.1 mL (class B) or 0.06 mL (class A). The difference between starting and final volume is the amount dispensed. The spacing between the lines will allow you to estimate the volume to the nearest 0.01 mL. Thus, typical burette readings would be two decimal points, for example, 9.34 mL or 17.60 mL. Reading such as 9.3 mL or 17.6 mL are not acceptable. The following are the step will help you to have a burette that operates as it should: a) b) c)

d)

Clean and rinse the burette with tap water, then distilled water. Rinse the burette with about 5-10 mL of the solution. Fill burette above the zero mark with the stop cork closed. Open the stopcock fully so that the liquid drain rapidly to flush out air bubble in the tip of the burette. Drain the burette until the meniscus rest at a certain number, for example, 1 mL marks (or 0 mL marks). Read the burette to 2 decimal places with your eye on the same level as the meniscus. To obtain the volume of the solution (liquid) that you use in titration, subtract the initial reading from the final reading. 2

To calibrate the burette, transfer several volumes of solution from the burette and weigh accurately. From the density of the solution, we can calculate the volume of solution that has been transferred. Table 1: Density of Water (g/mL) at Various Temperatures (°C) Temperature (°C)

Density of water (g/mL)

22

1.0032

23

1.0034

24

1.0037

25

1.0039

26

1.0042

27

1.0045

28

1.0047

29

1.0050

30

1.0053

35

1.0059

OBJECTIVES 1. 2.

To learn the qualitative and quantitative aspects of common laboratory equipment. To expose student to the factors that affect the accuracy of an experiment.

APPARATUS Analytical balance Burette Pipette (20 mL or 25 mL) Volumetric flask (25 mL) Beaker (50 mL) Thermometer Pipette filler or suction bulb Retort stand Burette clamp Dropper

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CHEMICAL Distilled water

PROCEDURE A.

Calibration of Volumetric Flask 1. 2.

3. 4. B.

Calibration of Pipette 1. 2. 3. 4. 5. 6.

C.

Clean and dry a 25 mL volumetric flask and weigh accurately using analytical balance. Record the mass of empty volumetric flask. Add distilled water until the calibration mark (use a dropper to add the last few drops of distilled water) and weigh again (use the same balance). Record the mass of distilled water and volumetric flask. Record the temperature of the distilled water. From the Table 1, determine the actual volume of the volumetric flask.

Clean and dry a 50 mL beaker and weigh accurately using analytical balance. Record the mass of empty beaker. Clean a pipette (20 or 25 mL) and rinse with distilled water. Fill the pipette with distilled water using the procedures that have been discussed in the introduction part. Drain the distilled water into the beaker and weigh again. Record the mass of distilled water and beaker. Repeat step 1-4 one more time and record the temperature of the distilled water. From the Table 1, determine the actual volume of the pipette.

Calibration of Burette 1. 2.

3. 4.

5. 6.

Clean and dry a 50 mL beaker and weigh accurately using analytical balance. Record the mass of empty beaker. Clean and rinse the burette using distilled water and fill in the burette with the distilled water until the zero mark. (Make sure there are no bubbles in the tip of the burette) Drain 5 mL of the water from the burette into the beaker and weigh as soon as possible. Record the mass. Repeat step 3 by draining water from the burette until the following burette reading become 10 mL, 15 mL and 20 mL. (Each time 5 mL distilled water has been added from the burette). Record the mass (distilled water + beaker) every time after adding 5 mL of water. Record the temperature of the distilled water. From the Table 1, determine the actual volume for every addition of 5 mL of distilled water. 4

QUESTIONS 1.

How do you overcome or reduce the problem of random error and systematic error while doing an experiment?

2.

In what situation do you use a volumetric flask, conical flask, pipette, and graduated cylinder? Explain your answer from the accuracy aspects of these apparatus.

3.

Explain how to read a burette. What are the factors to be considered while using the burette?

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DATASHEET EXPERIMENT 1 BASIC LABORATORY TECHNIQUES :

Name

:

Date

Student ID

:

Group :

RESULTS Data: a.

Calibration of Volumetric Flask Mass of empty volumetric flask (g) Mass of volumetric flask + distilled water (g) Mass of distilled water (g) Temperature of distilled water (oC) Density of water (from Table 1) (g/mL)

b.

Calibration of Pipette (i) Mass of empty beaker (g) Mass of beaker + distilled water (g) Mass of distilled water (g) Temperature of distilled water (oC) Density of water (from Table 1) (g/mL) 6

(ii)

c.

Calibration of Burette Mass of empty beaker (g)

: …………………..

Temperature of distilled water (oC)

: …………………..

Density of water (from Table 1) (g/mL)

: …………………..

After the addition of distilled water:

Reading of burette (mL)

Mass of beaker + distilled water (g)

Mass of distilled water (g)

5 10 15 20

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Mass of distilled water for each 5 mL burette reading (g)

Calculations: a.

Determine the actual volume of the volumetric flask based on calculation.

b.

Determine the actual volume of the pipette based on the calculation for experiment (i) and experiment (ii).

c.

Determine the actual volume of distilled water (in mL) for each 5 mL burette reading based on calculation.

Reading of burette (mL)

Volume of water (mL)

0-5 5-10 10-15 15-20

Lecturer’s signature,

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EXPERIMENT 2 DETERMINATION OF PERCENT COMPOSITION IN HYDRATE COMPOUNDS INTRODUCTION Several salts that occur naturally in nature or purchased from local retailer are hydrated salts. A hydrated salt is a salt which has a number of chemically bound water molecules attached to the salt within its crystalline structure. These water molecules maybe referred to as the waters of crystallization or water hydration. In most situations the number of moles of water will remain fixed as a function of the moles of salt present. The formula for a hydrated salt is represented by the formula for the anhydrous salt followed by a dot and the appropriate number of water molecules. The formula for copper (II) sulphate pentahydrate is CuSO 4.5H2O, which indicates that 5 waters of hydration are present for each 1 mole of CuSO 4 salt. Copper sulphate pentahydrate is a blue crystal, while anhydrous salt, meaning without water. CuSO4. 5H2O(s)

CuSO4 (s) + 5H2O (g) Δ

However, some salt have their water bound so tightly that producing an anhydrous salt is nearly impossible as in the case of iron (III) chloride hexahydrate. The salt would decompose before the anhydrous salt would be formed. The mass percentage of water in a hydrate can be determined by heating a known amount of a sample until complete dehydration is accomplished. Hydrate salt

Anhydrous salt + water Δ

The dehydration step will result in a lower mass reading, so it is possible to determine the amount of water that was present within the salt sample. Total mass of hydrate salt is a sum of mass of the anhydrous salt and mass of water of hydration. Total mass of hydrate salt = mass of the anhydrous salt + mass of waters of hydration The percent mass of water in the hydrate may also be easily calculated using a formula: Percentage of water = mass of water loss mass of sample

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x

100

OBJECTIVES 1. 2.

To calculate the mol of water (x) in barium chloride hydrate (BaCl2.xH2O) using percent composition concept and the atomic mass. To identify molecular formula of compound A from the thermal decomposition.

APPARATUS Crucible and lid Clay triangle Ring clamp Retord stand Bunsen burner Crucible tongs White tile

CHEMICALS Barium chloride hydrate (BaCl2.xH2O) Compound A (Compound A can be any of the following: Li2SO4.H2O, MgSO4.7H2O, FeSO4.7H2O, SrCl2.6H2O or CaSO4.2H2O) 6 M HNO3 1 M HCl

PROCEDURE To complete two trials in one lab period, your instructor may require you to perform both trials simultaneously. 1.

2. 3.

4.

Obtain a crucible and lid. Clean the crucible and check for any stress cracks, fractures or fissures before use. (If the crucible is dirty, move the apparatus to hood and add 1-2 mL of 6 M HNO3 and gently evaporate to dryness then inspect the crucible after cooling for any defects. If no defects are found the crucible and lid should supported on a clay triangle). Record an initial mass of the crucible and lid. Heat the crucible and lid gentle for 5 minutes with an intense flame until the bottom of the crucible has become red. Allow the crucible and lid to cool on the clay triangle before proceed the experiment. Determine the mass of the “fired” cool crucible and lid, then record. Repeat step 2 until you have two crucible and lid mass readings that differ by no more than 10 mg or 0.010 g. 10

5.

Setup the heating process as shown in Figure 1 below. Tilt the lid so that it is slightly ajar, then heat strongly (bottom of crucible should turn red) for about 5 minutes. Turn off the Bunsen burner, and use tongs to close the lid so that water from the air does not get inside the dry crucible. Allow the crucible and lid to cool (this should take 5 minutes).

Figure 1: Setup of the Heating Process 6. 7.

8.

9.

Add between 1.5 and 2.0 g of hydrated salt to the crucible and measure the combined mass of the crucible, lid and salt, then record. Place the crucible with the sample on the clay triangle. With the lid slightly ajar, heat the sample slowly for 2 minutes and drastically heat the sample at a high temperature for 10 minutes. Cover the crucible once the heat is removed and allow cooling at room temperature in desiccators. Record the mass of the crucible, lid and anhydrous salt using the same analytical balance as used in the earlier steps. Reheat the sample for an additional 2 minutes with intense heat. Weigh the combined mass of the crucible, lid and anhydrous salt and continue repeating this process until two concurrent reading within 10 mg of each other were obtained. Apply this procedure for Compound A.

DISPOSAL Dispose of all waste anhydrous salt in the “Solid Waste” container. CLEAN-UP Rinse the crucible with 2-3 mL of 1 M HCl and discard in the “Acid Waste” container. Rinse the crucible three times with tap water and once with distilled water.

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QUESTIONS 1.

Give the reason why the empty crucible should be heated before starting the experiment.

2.

Why the process of heating hydrate compound should be started slowly first?

3.

What is the important of percent composition of water in the hydrate compounds?

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DATASHEET EXPERIMENT 2 DETERMINATION OF PERCENT COMPOSITION IN HYDRATE COMPOUNDS Name

:

Date

:

Student ID

:

Group :

RESULTS Data: a.

Determination of Percent Water in Barium Chloride Hydrate (BaCl2.xH2O)

Mass of empty crucible and lid

g

First heating

g

Second heating

g

Mass of crucible + lid + hydrate before heating

g

Mass of crucible + lid + hydrate after, 1) First heating

g

2) Second heating

g

3) Third heating

g

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b.

Determination of Water Composition and Molecular Formula of Compound A.

Crucible and lid Mass of empty crucible and lid

g

First heating

g

Second heating

g

Mass of crucible + lid + hydrate before heating

g

Mass of crucible + lid + hydrate after, 1) First heating

g

2) Second heating

g

3) Third heating

g

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Calculations: a.

Determination of Percent Water in Barium Chloride Hydrate (BaCl2.xH2O) Calculate: 1. 2. 3. 4. 5.

b.

Mass of BaCl2.xH2O. Mass of barium chloride anhydrous. Mass of water in BaCl2.xH2O. Percent composition of water in BaCl2.xH2O. Formula of barium chloride hydrate.

Determination of Water Composition and Molecular Formula of Compound A. Calculate: 1. 2. 3. 4. 5.

Mass of Compound A. Mass of Compound A anhydrous. Mass of water in Compound A. Percent composition of water in Compound A. Identification of Compound A (compare your result to the nearest percent composition of water in the hydrates listed) Li2SO4.H2O, MgSO4.7H2O, FeSO4.7H2O, SrCl2.6H2O or CaSO4.2H2O

Lecturer’s signature,

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EXPERIMENT 5 INTRODUCTION TO TITRATION-DETERMINATION OF THE MOLARITY AND CONCENTRATION OF SULPHURIC ACID BY TITRATION WITH A STANDARD SOLUTION OF SODIUM HYDROXIDE INTRODUCTION Titration is a process in volumetric analysis that involved the reaction of analyte with a measured volume of reagent of known concentration. In this process, a solution of accurately known concentration, called a standard solution, is added gradually to another solution of unknown concentration until the chemical reaction between the two solutions is complete. An acid-base titration involves a neutralization reaction in which an acid is reacted with an equivalent amount of base. An indicator solution is used to determine the end point i.e. point at which an acid has exactly neutralized a base, or vice versa. A suitable colour change shows equivalent amounts of acids and base are present. Indicators change colours at different pH values. Phenolpthalein for example changes from colourless to pink at a pH of about 9. In this experiment, you will determine the molarity and concentration of sulphuric acid from data obtained by titrating it with a standard solution of sodium hydroxide until the end point. The molarity of sulphuric acid can be calculated by using the formula: MaVa M bV b

=

a b

where, Ma = concentration of acid Mb = concentration of base Va = volume of acid Vb = volume of base a = coefficient of acid b = coefficient of base The concentration of the sulphuric acid can be determined using the formula: Concentration (g/L) = Molarity x Molar mass

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OBJECTIVES To determine the molarity and concentration of sulphuric acid using titration technique.

APPARATUS Burette Burette clamp Retort stand Volumetric pipette (20 mL) Pipette filler or suction bulb Conical flask (250 mL) White tile / white paper

CHEMICALS...