Experiment 3 – Introduction to Weighing Technique and Density Determination Objectives 1. To learn to use an analytical balance. 2. To learn the weighing technique – weighing by taring. 3. To use proper weighing technique to determine the density of water, ethanol and hexane.

Apparatus: 1. 3- 50 mL Erlenmeyer flask and stopper 2. 10 mL graduated cylinder 3. Analytical Balance Solid: 1. Sand (for instructor demonstration) Liquids: 1. Distilled water 2. Ethanol (also named ethyl alcohol) 3. Hexane

CHEM 0011 – May 2010

Introduction The Balance Room Balances are sensitive to drafts, changes in temperature, or the vibrations caused by moving people. The balances are stored in a separate room to minimize these variables and are placed on concrete tables. Balances are very expensive and are sensitive to attack by corrosive chemicals. Do not take liquid into the balance room. When possible, chemicals should be added to the weighing container outside of the balance chamber. During the weighing process, the weighing container should be placed on a clean surface, such as a kimwipe, so that the bottom of the container does not pick up any dust. It is important that you clean up all chemical spills. If in doubt consult your instructor. Weigh boats are disposable containers used for weighing. They are made of polypropylene plastic and are inexpensive. A used weigh boat should be discarded in the waste container. All chemicals and spatulas that are used should be returned to their proper places. Depending on the experiment, other types of weighing container could be a porcelain crucible, an aluminum container or a small beaker.

The balance room must be kept tidy. Materials taken into the balance room include datasheets, pen and the sample to be weighed. Enter mass CHEM 0011 – May 2010

measurements directly on the datasheet with your pen. Before you leave your balance, make sure: 1. The balance and the area around it is clean. Spills inside the balance should be brushed off using the brush on top of the balance. Spills on the concrete table should be cleaned using Kimwipes. 2. Close all the doors of the balance. 3. Turn off the balance.

CHEM 0011 – May 2010

Part A - Technique on Weighing by Taring Introduction There are many different types of balances or scales available to measure the mass of a sample. The selection of the balance depends on the mass of the object or sample and the precision needed for the measurement. A top-loading balance is used to determine the approximate mass of the sample needed. In this course, we will be mostly using electronic analytical balances. These balances are easy to operate and are capable of measuring to 0.0001 g. Models of these balances vary in the labs. Consult your instructor and operating manual specific to the model of the balance.

In this course we will focus on the weighing by taring technique. This technique is used when the mass of the empty container is not important. The empty container is ‘zeroed’ or tared on the balance.

CHEM 0011 – May 2010

Part A - Instructor Demonstration of Weighing by taring Procedure: 1. Place a weigh boat on the balance pan. Close the doors and wait for the reading to stabilize. Press briefly on the control bar or the tare button and the display changes to 0.0000 g. The weight of the weighing boat is now tared. 2. Remove the weigh boat from the balance and set it on a piece of kimwipe. With a spatula, carefully add the sample to the weigh boat. Place the weigh boat back on the balance pan. Close the doors and wait for the reading to stabilize. Record the mass of the sample.

Part A - Datasheet for Weighing by taring

1. Mass of weigh boat and sample

______________________ g

2. Mass of empty weigh boat

3. Mass of sample

0.0000 g

______________________ g

Question: 1. How many significant figures does the mass of the sample have?

2. How many significant figures does the mass of the tared empty weigh boat have?

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Part B – Density Determination Introduction Density is a physical property, which can be used to identify substances. Density is defined as the mass per unit of volume. Density = mass/volume Mass is a measure of the quantity of matter contained in an object. Volume is a measure of the space occupied by an object.

Part B: Density Determination (to be carried out with a lab partner) Procedure 1. Measure about 10 mL of distilled water with a 10 mL graduated cylinder. 2. Turn on the balance and tare a clean and dry 50 mL Erlenmeyer flask with a rubber stopper on the balance. 3. Remove the tared Erlenmeyer flask from the balance and transfer the distilled water from the graduated cylinder to the Erlenmeyer flask outside the balance room. Put the stopper on the Erlenmeyer flask to avoid evaporation. Do not turn off the balance. 4. Return to the balance that was used in step 3, and weigh the Erlenmeyer flask, distilled water and stopper. 5. Turn off the balance. 6. Record the mass of the Erlenmeyer flask, distilled water, and stopper on the datasheet. 7. Record accurately the volume of the distilled water that was transferred into the Erlenmeyer flask to the proper number of significant figures. 8. Repeat the above steps 1 to 7 for ethanol and hexane. NOTE: Avoid inhaling hexane vapour. It is toxic. To dispose of the hexane, a waste container is provided.

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Datasheet: Part A: Density Water: 1. Mass of Erlenmeyer flask + distilled water + stopper

_______________ g

2. Mass of empty Erlenmeyer flask + stopper 3. Mass of distilled water

0.0000 g __________________ g

4. Volume of distilled water measured in graduated cylinder

___________ mL

5. Volume of distilled water remained in graduated cylinder ______________ mL

6. Volume of distilled water transferred

______________ mL

Ethanol: 1. Mass of Erlenmeyer flask + ethanol + stopper 2. Mass of empty Erlenmeyer flask + stopper 3. Mass of ethanol

_______________ g 0.0000 g __________________ g

4. Volume of ethanol measured in graduated cylinder

___________ mL

5. Volume of ethanol remained in graduated cylinder

______________ mL

6. Volume of ethanol transferred

______________ mL

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Hexane: 1. Mass of Erlenmeyer flask + hexane + stopper

_______________ g

2. Mass of empty Erlenmeyer flask + stopper 3. Mass of hexane

0.0000 g ______________________ g

4. Volume of hexane measured in graduated cylinder

___________ mL

5. Volume of hexane remained in graduated cylinder

______________ mL

6. Volume of hexane transferred

______________ mL

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Sample Density Calculation: Show work here for Ethanol:

Summary of Densities Measured: Room Temperature = ___________________________________

Density of Liquid

Water

Ethanol

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Hexane

Percent Error Calculation The percent error is defined mathematically by equation: % Error = |accepted value - experimental value| * 100 accepted value Given that the accepted value for the density of ethanol is 0.79 g/mL, calculate the % error in your result. Show work here and report your answer with proper number of significant figures.

Question:

1. Do you think temperature affects the density of the liquid. Explain your answer.

CHEM 0011 – May 2010