experiment #3 using physical properties determine...

Experiment #3– Using Physical Properties To

Determine the Identity of an Unknown

Laboratory Overview

CHEM 1361

August 2012

Gary S. Buckley, Ph.D. Department of Physical Sciences

Cameron University

Learning Objectives

•Distinguish between physical and chemical properties •Experimentally determine the boiling point of a compound •Determine the index of refraction of a material, time and instrumentation permitting •Experimentally determine the miscibility of two liquids •Use physical property data to identify an unknown from a list of possible compounds

Table of Contents (you may click on any of the topics below to go directly to that topic)

•Density •Boiling Point •Index of Refraction •Solubility

Density

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Density is defined to be the mass of a sample of a substance divided by its volume. In mathematical terms, In this experiment, you will determine the density of your unknown as a means of helping determine its identity. The measurement is a simple matter of determining the mass of a known volume and carrying out the division above.

massdensity

volume

Density’s Role in this Experiment

Density will be used in this experiment as one of the physical properties to help identify your unknown from the list at the right. There are a couple of important things to notice: •If only one significant figure is measured experimentally, 5 of the 9 compounds would have the same density (0.8 g/mL) •The instructions for measuring the density will lead to masses and volumes to at least three significant figures. Your density values should all have three significant figures as well as your average. Don’t be lazy with significant figures – remember that if you divide , for example, 2.94 g by 1.47 mL, the density is 2.00 g/mL – not 2 g/mL.

Possible Unknowns

Compound Density

(g/mL)

water 1.00

ethyl acetate 0.902

ethanol (95%) 0.816

1-propanol 0.804

methanol 0.791

Acetone 0.791

2-propanol 0.785

cyclohexane 0.779

hexane 0.659


Boiling Point

The liquid state of matter is comprised of atoms or molecules that are virtually in constant contact with each other moving relatively slowly past each other. The vapor state on the other hand contains atoms or molecules that are spaced far away from each other on the atomic scale and are moving rapidly with frequent collisions. In any liquid sample atoms or molecules are constantly escaping into the vapor phase and condensing into the liquid phase. As the temperature of the liquid increases, the fraction of molecules in the vapor phase increases. The pressure of the vapor above the sample is called the vapor pressure of the liquid. The temperature at which the vapor pressure of the sample equals the external pressure on the sample is called the boiling point. The temperature at which the vapor pressure of the liquid is 1 atmosphere is called the normal boiling point.


Boiling Point

Temperature

Low

Pressure

Low

Low Temperature

Temperature

Med

Pressure Med.

Medium Temperature

Temperature

High

Pressure

High

High Temperature

The animation on the right is intended to give some visual sense of the effect of temperature on the vapor pressure of a liquid. Watch the temperature and pressure gauges and the distribution of atoms/molecules between the liquid and vapor phases. Click anywhere on this slide to start the animation.

Boiling Point - Experimental

The boiling point of your unknown will be determined by heating a test tube containing the unknown in a temperature bath. A stainless steel temperature probe connected to a handheld LabQuest 2 data acquistion device will be placed just above the surface of your unknown. As the unknown is heated the temperature will rise but it will level off once the unknown begins to boil. Once the temperature levels off, you will find the average temperature in the leveled-off region and record that as your boiling point. Instructions for using the LabQuest 2 may be found under another item in Blackboard in the same location as this slide show. It is a 20-minute video showing you how to work the LabQuest 2.

Index of Refraction


You are probably aware that light travels incredibly fast – 3 ×

108 m/s or 186000 miles/s. These speeds refer to light

traveling in a vacuum – it travels more slowly in other

materials.

The index of refraction, η, is the ratio of the speed of light in a

vacuum to the speed of light in the material of interest.

Since light travels fastest in a vacuum, η will always be

greater than one. We can use the value of η to help

distinguish between the possible unknowns.

speed of light in vacuum

speed of light in substance of interest

Possible Unknowns

Compound

nD

Refractive

index

Cyclohexane

1.426

1-propanol

1.384

2-propanol

1.377

Hexane

1.375

ethyl acetate

1.372

ethanol (95%)

1.363

Acetone

1.359

Water

1.333

Methanol

1.329

Index of Refraction


You may or may not have an opportunity to carry out index of refraction measurements – it depends on instrumentation availability the day you do the lab. The instrument used to measure the index of refraction is called a refractometer. A liquid sample is placed on the stage and the cover is gently closed. If the switch on the left-hand side of the instrument is moved up, a light can be seen through the ocular part of the refractometer. (Description continued on next slide)

Switch

Stage

Ocular Piece

Index of Refraction


The goal is to turn the knob on the right-hand side of the instrument until the light-dark horizontal separation in the ocular is aligned at the crosshairs. Once this positioning is accomplished, the switch on the left-hand side is moved to the downward position and the index of refraction reading is made on the visible scale. The reading in the window to the right would be 1.3596. The lower scale is not used here.

Solubility


Solubility is the fourth of the physical properties you will investigate. Two liquids that mix with each other are said to be soluble, or miscible. If the liquids do not mix with each other they are said to be insoluble, or immiscible.

Possible Unknowns

Compound

solubility

in water

solubility in

cyclohexane

ethyl acetate sl. s s

water s i

1-propanol s s

2-propanol s s

ethanol (95%) s sl. s

methanol s sl. s

acetone s s

cyclohexane i s

hexane i s

As the table to the right indicates, the solubility behavior of the unknowns is not a great differentiator between the compounds. But, when coupled with the other information, these data may be helpful in confirming your unknown identification. You will check the solubility of your unknown in two solvents – water and cyclohexane. Pay careful attention to the instructions in the lab manual regarding looking for wavy lines and the behavior following stirring.

End of Slide Show

experiment #3 using physical properties determine...

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