Chemistry: Applied and Descriptive

Labs & Worksheets Features

Teacher's Manual

Labs & Worksheets

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• 469 pages, 3-hole punched, shrink-wrapped, consumable
• Includes the Exercise questions from the Student Edition, as well as Labs and Demos
• Organized into Units and sections to correlate to the Student Edition
• Included on the Teacher Component disc as Word files

Below is an excerpt from Unit J of the Labs & Worksheets.

1. Define the following terms.
   a) solute

   b) solvent

   c) solution

   d) concentration

2. What units are used to measure molar concentration?

1. Copper(II) sulfate, an important copper salt, is used in copper electroplating cells, and to kill algae in swimming pools and water reservoirs. What is the molar concentration of an electroplating solution in which 1.50 mol of copper(II) sulfate are dissolved in enough water to make 2.00 L of solution?
   
   
2. When 11.0 g of glacial (pure) acetic acid is dissolved in water to make 250 mL of vinegar solution, what is the molar concentration of the vinegar?
   
   
3. Sodium bicarbonate is used medicinally to counteract excess stomach acidity. How many moles of solid sodium bicarbonate would be needed to make 100 mL of a 0.660 mol/L solution suitable for use as an antacid?
   
   
4. A toilet bowl cleaner may be prepared by mixing sodium bicarbonate (baking soda) and sodium hydroxide (lye). What mass of sodium bicarbonate must be added to a 2.50 L bowl to obtain a necessary 0.150 mol/L solution?
   
   
5. Sodium phosphate may be used to remove scale deposits from a car radiator. What volume of a 0.075 mol/L solution would contain the necessary 1.10 mol of sodium phosphate to remove the radiator scales?
   
   
6. Chlorine bleach in its solution form usually is sold as a 5 to 6 percent solution of sodium hypochlorite; e.g., as in Clorox and Purex. How many liters of 0.800 mol/L solution would contain 119.2 g of NaOCl?

J.6 Demo: Preparation of a Solution

Chemists and technicians must constantly prepare solutions. As techniques and instrumentation become more complex, the need for proper methods becomes more essential if accuracy is to be maintained.

Purpose:

• To prepare 100.0 mL of 0.0350 mol/L aqueous solution of NiCl₂ x 6H₂O.
• To demonstrate correct techniques for the preparation of a solution.

Materials:

100-mL volumetric flask 250-mL beaker funnel (short stem) stirring rod centigram balance wash bottle

scoopula distilled water NiCl2 x 6 H2O (or another colored compound) meniscus finder eyedropper

Predemo Information:
In general, the steps to be followed in preparing a solution are:

1. Calculate the mass of solute required.
2. Use a balance to obtain the required mass of solute.
3. Dissolve the required mass of solute in less than the final volume of water.
4. Transfer the solution to a volumetric flask.
5. Bring the solution up to final volume.
6. Stopper and invert several times to mix.

(Note: These steps are outlined in more detail in the procedure below.)

Predemo Exercise:

• Calculate the mass of solute required to prepare 100.0 mL of a 0.0350 mol/L aqueous solution of NiCl₂ x 6 H₂O.

Procedure:

1.     a.   Obtain and record mass of a clean dry 250 mL beaker to 0.01 g.
   2. Make a note of the mass of a beaker plus the required mass of solute.
      
   3. Add NiCl₂ x 6 H₂O until the desired amount of solid is obtained. (Use a tapping action on the scoopula to control the addition of the NiCl₂ x 6 H₂O.)
      


2. Add about 60 mL of distilled water to the solute in the beaker. Stir to get the solute to dissolve more rapidly. (When the stirring rod is removed, use a wash bottle containing distilled water to rinse the solution from the stirring rod into the solution in the beaker.)
   


3.     a.   Put a clean, short-stemmed funnel into a clean, 100-mL volumetric flask.
   
   2. Pour the solution from the beaker through the funnel into the volumetric flask. (When pouring, hold the stirring rod onto the lip of the beaker to avoid loss of some solution down the side of the beaker.)
   3. Use the wash bottle to rinse any remaining solution from the beaker, stirring rod and funnel. Do not use an excessive amount of wash water. Remove the funnel from the volumetric flask. (The solution should not have been increased in volume to the point where it was touching the stem of the funnel.)


4.     a.   Use distilled water from the wash bottle to bring the solution volume up to just below the 100.0-mL line on the volumetric flask.
   
   2. Use an eyedropper, distilled water and meniscus finder to bring the bottom of the meniscus up to the 100-mL line on the volumetric flask. (The dark line on the meniscus finder should be kept just barely below the meniscus in order to get a black meniscus against a white background.)
   3. Stopper the volumetric flask. Mix the solution thoroughly by inverting (not shaking) the volumetric flask several times. Leave the volumetric flask stoppered.

Questions:

1. Look at the markings on a 100-mL volumetric flask. Note the calibration line on the neck and where “TC 20°C” is written on the flask. This means that the flask is calibrated To Contain the correct amount of liquid at room temperature (20°C). Explain how these markings are important.
   
2. Why was a volumetric flask used rather than a graduated beaker or graduated cylinder?
   
3. Which parts of the following materials had to be dry when initially used in preparation of the solution? List the following materials under the appropriate heading: NiCl₂ x 6 H₂O, scoopula, 250-mL beaker, stirring rod, funnel, volumetric flask and the volumetric flask stopper.

   Had to be dry

   Could be wet

4. Why was it necessary to rinse the equipment in step 3 and step 4?
   
5. Why was the final solution mixed?
   
6. Why should the final solution be left stoppered?