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Introduction

Specific gravity is relative value being as

usually determined the weight of definite

volume of substance at known temperature

compared with the weight of similar volume of

some other substance also at known

temperature; it is therefore a ratio. In the case

of liquids, the standard substance universally

adopted for the ratio is water. [1] If a substance

has a specific gravity less than one it will float

on water. If the specific gravity is more than one

it will sink. Specific gravity has no units. Specific

gravity values are the same whether calculated

from metric or English densities since the ratio

of the substance densities remains the same.

Specific gravity depends slightly on

temperature. A change in temperature has no

effect on the mass of an object since the total

amount of matter present does not change. The

objects volume, however, does change when

the temperature increases because thermal

motion moves molecules further apart.

Therefore, for all three states of matter, anincrease in temperature generally causes

specific gravity to decrease. A properly reported

specific gravity value should therefore include

the temperature at which the measurement

was made. When no temperature is given,

assume room temperature, 20 or 25C. [2]

There are different methods to

determine the specific gravity of liquid

substances. There is the pycnometer method

that uses the leach pycnometer which is

specifically used for liquids, the floatation

method that follows the Archimedes principle,

the manometric method which makes use of

the Fisher-Davidson Gravitometer, and many

more. The method used in this experiment is

the floatation method by making use of the

Mohr-Westphal Balance.

The Westphal Balance operates by

suspending a glass tube into a sample of a

solution of unknown density via a thin platinumwire. The scale relies on Archimedes'

Principle of buoyancy and is balanced by an

array of horseshoe shaped counterweights

which come in 5 g, 0.5 g, 0.05 g, and 0.005 g

masses. These counterweights, sometimes

called riders, respectively signify the ones

place of the specific gravity of the sample

solution, the tenths place, the hundredths, and

the thousandths place. The numerical value

each rider represents is equal to the numberednotch of the arm which it sits in when the scale

is balanced. To operate a Westphal Balance

care must be taken to first calibrate the balance

by means of the levelling screw at the bottom

of the body. With no weight on the arm of the

balance the two pointers must be aligned

before the balance can be used. Since the

Westphal Balance is measuring specific gravities

of our sample we can proceed to divide the

numerical result of all samples' specific gravitiesby density of the ref (the density of water at 4oC

in a vacuum, density of ref = 0.999973 g/cm3) in

order to get the density of sample. It is in this

way that we ensure we are finding the exact

density of each sample we examine. This step

may be skipped, however, if the density of

water at current air conditions is assumed to be

1 g/cm3. [3]

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Figure 1: Mohr-Westphal Balance: (1) balance beam, (2)

transverse notch scale, (3) stationary counterweight, (4)

balance indicator scale, (5) glass float, (6) riders, (7) vessel

with liquid, (8) thermometer, (9) double pan ([a] upper

pan, solid brass; [b] lower

pan, made of glass or aluminum, with openings)

There are also different methods in

determining the specific gravity of solidsubstances. Examples are the hydrostatic

balance method, the specific gravity or the

pycnometer method that is used for samples in

powder or small granule form, the graduated

cylinder method which is based on the

displaced volume of the substance from the

standard liquid, etc. The method used in this

experiment is the pycnometer method.

A pycnometer is a small glass bottle of

known volume for determining the relative

density of liquids and solids. The mass of an

irregular solid is determined by weighing. When

the solid is placed in a pycnometer filled with a

liquid of known density, the volume of the

liquid which will overflow is equal to the volume

of the solid. The mass of the liquid which will

overflow is determined as the difference

between the sum of the mass of the

pycnometer filled with liquid plus the mass of

the solid and the mass of the pycnometer filled

with liquid after the solid has been placed

inside. The volume occupied by this mass is

determined from the known density of the

liquid. It is necessary that the solid be insoluble

in the liquid used. The density of the solid is

determined from these measurements of mass

and volume. [4]

Figure 2: Pycnometer

The objectives of this experiment are:

to determine the specific gravity of the assignedliquid acetone by using the Mohr-Westphal

balance and to determine the specific gravity of

the assigned solid acetanilide by using the

pycnometer method.

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[1] (1919), Methods of Determining Specific

Gravity of Liquids. Jnl Institute Brewing,

25: 209210. Retrieved from:

http://onlinelibrary.wiley.com/doi/10.1002/j.20

50-0416.1919.tb04791.x/pdf

[2]

http://facultywp.ccri.edu/eterezakis/files/2013/

06/1180-Exp-04-Density-and-Specific-

Gravity.pdf

[3]

http://everything2.com/title/Westphal+Balance

[4]

http://blog.cencophysics.com/2009/07/density-

liquids-solids-pycnometer-method/

http://facultywp.ccri.edu/eterezakis/files/2013/06/1180-Exp-04-Density-and-Specific-Gravity.pdfhttp://facultywp.ccri.edu/eterezakis/files/2013/06/1180-Exp-04-Density-and-Specific-Gravity.pdfhttp://facultywp.ccri.edu/eterezakis/files/2013/06/1180-Exp-04-Density-and-Specific-Gravity.pdfhttp://facultywp.ccri.edu/eterezakis/files/2013/06/1180-Exp-04-Density-and-Specific-Gravity.pdfhttp://everything2.com/title/Westphal+Balancehttp://everything2.com/title/Westphal+Balancehttp://everything2.com/title/Westphal+Balancehttp://facultywp.ccri.edu/eterezakis/files/2013/06/1180-Exp-04-Density-and-Specific-Gravity.pdfhttp://facultywp.ccri.edu/eterezakis/files/2013/06/1180-Exp-04-Density-and-Specific-Gravity.pdfhttp://facultywp.ccri.edu/eterezakis/files/2013/06/1180-Exp-04-Density-and-Specific-Gravity.pdf