Exp.1 The Solubility of a Salt PDF

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CHEM 1103 2021

Experiment 1

EXPERIMENT 1 An Introduction to Data Collection: The Solubility of a Salt Objectives  Become familiar with data collection and analysis using a temperature probe and LabQuest data collection software. 

Collect the experimental data necessary to construct a solubility curve for potassium chloride in water.

Background Solubility is an important physical property in chemistry which describes the amount of solute that dissolves in a given amount of solvent. The solubility of chemical compounds varies extremely and can be affected by factors such as pressure, temperature, pH, and intermolecular forces or interionic forces in both the solute and the solvent. For most ionic compounds, solubility varies directly with temperature, thus, solubility is often expressed as the mass of solute that dissolves in 100 g of water at a particular temperature. A saturated solution is a solution that contains the maximum amount of dissolved solute at a given temperature. In this type of solution dissolution and precipitation are occurring at the same rate, thereby satisfying the requirement for a dynamic equilibrium. Two statements can be made about this solution: (1) the two processes, dissolution and precipitation, are going on at the same time, and (2) the number of molecules or ions in the solution remains constant. For example, the equilibrium of sodium chloride with its ions in a saturated solution would be shown by the equation NaCl(s)

Na+(aq) + Cl-(aq)

An unsaturated solution contains less solute than does a saturated solution, i.e., no equilibrium is present. When additional solute is added to an unsaturated solution, it dissolves. When additional solute is added to a saturated solution, the amount of dissolved solute does not increase, because the limit of solubility has already been reached. A super saturated solution contains more than the maximum amount of dissolved solute than a saturated solution under the same conditions. Since solubility often varies with temperature a solution that is saturated at one temperature may be unsaturated at a different temperature. As well, temperature changes may cause a saturated solution to become super saturated. In this experiment, you will collect data using temperature probe and LabQuest interface to investigate the effect of changing temperature on the amount of potassium chloride (KCl) that will dissolve in a given amount of water. You will completely dissolve different known quantities of KCl in the same volume of water at a high temperature. As each solution cools, you will monitor temperature and record the precise temperature at which solid crystals start to form. At this moment, the solution is saturated and contains the maximum amount of solute at that temperature. Each data pair consists of a solubility value (g of solute per 100 g H 2O) and a corresponding temperature. A graph of the temperature-solubility data, known as a solubility curve, can be obtained. Solubility curves are used to 1 of 7

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Experiment 1

predict the mass of solute in water at any given temperature. Typical solubility curves are shown in Figure 1.1. Here solid lines represent saturated solutions. Unsaturated solutions exist below the solid line, whereas super saturated solutions exist above the solid line.

Figure 1.1 Typical Solubility Curves Glassware & Equipment      

Balance Beakers: 250 & 400 mL Graduated cylinder: 10 mL Hot plate Retort stand Stirring rod

     

Test tubes: 5 (18 x 150 mm) Test tube rack Utility clamps (2) Vernier LabQuest interface Vernier Temperature probe Weighing funnel and spatula

Chemicals 

Potassium chloride (KCl)

Procedure 1.

Power-on the LabQuest unit (top left power button). The LabQuest App launches automatically. Connect a temperature probe to the CH 1 port. The LabQuest App will auto-ID the connected sensor and a live reading will appear in the Meter screen.

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2.

Set up the data-collection mode: On the Meter screen, tap Mode. Change the mode to Events with Entry. Enter the Name (Solubility) and Units (g/100g). Select OK.

3.

Fill a 400 mL beaker three-fourths full of tap water. Place it on a hot plate situated on (or next to) the base of a retort stand (watch for hot plate power cord not to touch the hot surface). Heat the water bath to about 90°C and adjust the heat to maintain the water at this temperature. Place the temperature probe in the water bath to monitor the temperature and to warm the probe. To keep from damaging the temperature probe wire, hang it over another utility clamp pointing away from the hot plate, as shown in Figure 1.2.

Figure 1.2 Equipment Set-up 4.

Use the weighing funnel, measure out the amount of KCl required for the first test tube as shown in the second column of Table 1.1 from your Data Sheets (amount per 5 mL). Record the exact mass with the correct number of decimal places. Transfer to a clean and dry test tube.

5.

Add precisely 5.0 mL of distilled water to the test tube, measured from a 10-mL graduated cylinder. Note: The fourth column in Table 1 (amount per 100 g of H 2O) is proportional to your measured quantity and is the amount you will enter for your graph in Step 7. Calculate this value before proceeding. Assume 1.0 g/mL for the density of water.

6.

Use a utility clamp to fasten the test tube to the retort stand. Lower the test tube into the water as shown in Figure 1.2. Use a stirring rod to stir the mixture until the KCl is completely dissolved. Note: In order to dissolve all the KCl, Test Tubes 3 and 4 need to be heated to a higher temperature than Test Tubes 1 and 2.

7.

You are now ready to collect temperature-solubility data: i) When the salt is completely dissolved, remove the temperature probe from the water bath, wipe it dry, and place it into the solution in the test tube. ii) Tap Collect

to start the data collection.

iii) Unfasten the utility clamp holding the test tube from the ring stand. Use the clamp to hold the test tube up to the light to look for the first sign of crystal formation. At the same time, stir the solution with a slight up and down motion of the temperature probe. Note: Test tubes 1 and 2 may be cooled to lower temperatures using cool tap water in a 250 mL beaker. This drops the temperature much faster than air. If the crystals form too quickly, briefly warm the test tube in the hot-water bath and re-dissolve the solid. Then repeat the cooling and collect the data pair. 3 of 7

CHEM 1103 2021

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iv) Now crystallization starts to occur, tap Keep . Enter the mass in grams (the solubility value in column 3 of Step 1, g per 100 g H2O). Select OK to store the temperature-mass data pair. v) After you have saved the temperature-mass data pair, return the test tube to the test tube rack and place the temperature probe in the water bath for the next trial. 8.

Repeat Steps 4 – 7 for each of the other four samples listed in Table 1.1.

9.

When you have collected the last data pair, stop data collection by pressing the red stop button (lower left corner of the screen).

10.

Data for columns 3 and 5 in Table 1.1 will be provided. Record these values in your Data Sheets.

10.

Use the provided grid in the Data Sheets to plot solubility (grams KCl/ 100 g water) on the Y-Axis and the corresponding temperature on the X-Axis. Add a title to your plot and correctly label the axes.

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Name:

Section:

REPORT SHEETS – EXPERIMENT 1 An Introduction to Data Collection: The Solubility of a Salt Data and Results [2.5] Table 1.1

KCl (g) per 100 g H2O Test tube KCl (g) per Actual mass of KCl (g) (show all calculation number 5 mL H2O per 5 mL H2O below the table) 1

1.7

2

1.9

3

2.1

4

2.3

5

2.5

Temp. (°C)

Calculations: (1.0)

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Experiment 1 Graph (1.5)

Eqn. of line: Name:

Correlation coefficient = Section:

Data Analysis [12.5] 1.

According to your data, how is solubility of KCl affected by the temperature of the solvent? (1.0)

2.

Looking at your graph, indicate whether each of these solutions would be saturated, unsaturated, or super saturated. (1.5) a. 44 g of KCl in 100 g of water at 60°C __________________________ b. 80 g of KCl in 100 g of water at 85°C __________________________ c. 15 g of KCl in 50 g of water at 30°C __________________________

3.

According to your graph, will 20 g of KCl completely dissolve in 100 g of water at 20°C? Explain. (1.5)

4.

Assuming a linear relationship, use the equation of the line to calculate: a. The amount of KCl that will dissolve in 100 g of water at 50°C. (1.5)

b. The amount of KCl required to saturate 1.75 kg of water at 101°C. (1.5)

c. The minimum temperature required to completely dissolve 1.05 moles of KCl in 180 g of water. (2.0)

d. What is the minimum amount of water (in grams) required to completely dissolve 60.0 g of KCl at 90°C? (1.5)

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e. A KCl saturated solution is cooled from 80°C to 45°C, calculate the quantity of crystals that will form once crystallization starts. (2.0)

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