Density

Submitted by ChemPRIME Staff on Wed, 12/08/2010 - 23:35


The terms heavy and light are commonly used in two different ways. We refer to weightA measure of the gravitational force on an object; directly proportional to mass. when we say that an adult is heavier than a child. On the other hand, something else is alluded to when we say that oak is heavier than balsa wood. A small shaving of oak would obviously weigh less than a roomful of balsa wood, but oak is heavier in the sense that a piece of given size weighs more than the same-size piece of balsa.

What we are actually comparing is the massA measure of the force required to impart unit acceleration to an object; mass is proportional to chemical amount, which represents the quantity of matter in an object. per unitA particular measure of a physical quantity that is used to express the magnitude of the physical quantity; for example, the meter is the unit of the physical quantity, length. volume, that is, the densityThe ratio of the mass of a sample of a material to its volume.. In order to determine these densities, we might weigh a cubic centimeter of each type of wood. If the oak sample weighed 0.71 g and the balsa 0.15 g, we could describe the density of oak as 0.71 g cm–3 and that of balsa as 0.15 g cm–3. (Note that the negative exponent in the units cubic centimeters indicates a reciprocal. Thus 1 cm–3 = 1/cm3 and the units for our densities could be written as \tfrac{\text{g}}{\text{cm}^{3}}, g/cm3, or g cm–3. In each case the units are read as grams per cubic centimeter, the per indicating division.) We often abbreviate "cm3" as "cc", and 1 cm3 = 1 mL exactly by definition.


In general it is not necessary to weigh exactly 1 cm3 of a material in order to determine its density. We simply measure mass and volume and divide volume into mass:


\text{Density = }\frac{\text{mass}}{\text{volume}}\text{      or     }\rho \text{ = }\frac{\text{m}}{\text{V}}\text{                                                (1)}


where ρ = density      m = mass      V = volume



EXAMPLE 1 Calculate the density of (a) a piece of aluminum whose mass is 37.42 g and which, when submerged, increases the water level in a graduated cylinder by 13.9 ml; (b) an aluminum cylinder of mass 25.07 g, radius 0.750 cm, and height 5.25 cm.


SolutionA mixture of one or more substances dissolved in a solvent to give a homogeneous mixture.

a) Since the submerged metalAn element characterized by a glossy surface, high thermal and electrical conductivity, malleability, and ductility. displaces its own volume,

\text{Density}=\rho =\frac{\text{m}}{\text{V}}=\frac{\text{37}\text{.42 g}}{\text{13}\text{.9 ml}}=\text{2}\text{.69 }{\text{g}}/{\text{ml or 2}\text{.69 g ml}^{-1}}\;


b) The volume of the cylinder must be calculated first, using the formula


V = π r2h = 3.142 × (0.750 cm)2 × 5.25 cm = 9.278 718 8 cm3


Then \rho =\frac{\text{m}}{\text{V}}=\frac{\text{25}\text{.07 g}}{\text{9}\text{.278 718 8 cm}^{3}} \left. \begin{align}
  & \text{ = 2}\text{.70}\frac{\text{g}}{\text{cm}^{3}} \ 
 & =\text{2}\text{.70 g cm}^{-3} \ 
 & =\text{2}\text{.70 }{\text{g}}/{\text{cm}^{3}}\; \ 
\end{align} \right\}\text{all acceptable alternatives}



Note that unlike mass or volume (extensive properties), the density of a substanceA material that is either an element or that has a fixed ratio of elements in its chemical formula. is independent of the size of the sample (intensive propertyA property for which the value does not depend on the quantity of matter under consideration. Density is an example of an intensive property; mass and volume are not intensive properties.). Thus density is a property by which one substance can be distinguished from another. A sample of pure aluminum can be trimmed to any desired volume or adjusted to have any mass we choose, but its density will always be 2.70 g/cm3 at 20°C. The densities of some common pure substances are listed below.

Tables and graphs are designed to provide a maximum of information in a minimum of space. When a physical quantity (number × units) is involved, it is wasteful to keep repeating the same units. Therefore it is conventional to use pure numbers in a table or along the axes of a graph. A pure number can be obtained from a quantity if we divide by appropriate units. For example, when divided by the units gram per cubic centimeter, the density of aluminum becomes a pure number 2.70:


\frac{\text{Density of aluminum}}{\text{1 g cm}^{-3}}=\frac{\text{2}\text{.70 g cm}^{-3}\text{ }}{\text{1 g cm}^{-3}}=\text{ 2}\text{.70}

Density of Several Substances at 20°C.

Substance Density / g cm-3
Helium gas 0.000 16
Dry air 0.001 185
Gasoline 0.66-0.69 (varies)
Kerosene 0.82
Benzene 0.880
Water 1.000
Carbon tetrachloride 1.595
Magnesium 1.74
Salt 2.16
Aluminum 2.70
Iron 7.87
Copper 8.96
Silver 10.5
Lead 11.34
Uranium 19.05
Gold 19.32


Therefore, a column in a table or the axis of a graph is conveniently labeled in the following form:

Quantity/units


This indicates the units that must be divided into the quantity to yield the pure number in the table or on the axis. This has been done in the second column of the table.



Converting Densities

In our exploration of Density, notice that chemists may express densities differently depending on the subject. The density of pure substances may be expressed in kg/m3 in some journals which insist on strict compliance with SI unitsThe international system of units (Système International d'Unité) based on seven fundamental units: meter, kilogram, second, ampere, kelvin, candela, mole.; densities of soils may be expressed in lb/ft3 in some agricultural or geological tables; the density of a cell may be expressed in mg/µL; and other units are in common use. It is easy to transform densities from one set of units to another, by multiplying the original quantity by one or more unity factors:


EXAMPLE 2 Convert the density of water, 1 g/cm3 to (a) lb/cm3 and (b) lb/ft3


Solution a. The equality 454 g = 1 lb can be used to write two unity factors,

\frac{454 g}{1 lb}    or   \frac{1 lb}{454 g}.

The given density can be multiplied by one of the unity factors to get the desired result. The correct conversion factorA relationship between two units of measure that is derived from the proportionality of one quantity to another; for example, the mass of a substances is proportional to its volume and the conversion factor from volume to mass is density. is chosen so that the units cancel:

1  \frac {g}{cm^3} x \frac{1 lb}{454 g}   =  0.002203 \frac{lb}{cm^3}

b. Similarly, the equalities 2.54 cm = 1 inch, and 12 inches = 1 ft can be use to write the unity factors:

\frac{2.54 cm}{1 in},      \frac{1 in}{2.54 cm},     \frac{12 in}{1 ft}     and    \frac{1 ft}{12 in}

In order to convert the cm3 in the denominator of 0.002203 \frac{lb}{cm^3} to in3, we need to multiply by the appropriate unity factor three times, or by the cube of the unity factor:

0.002203   \frac{g}{cm^3} x \frac{2.54 cm}{1 in} x \frac{2.54 cm}{1 in} x \frac{2.54 cm}{1 in}


or 0.002203  \frac{g}{cm^3} x (\frac{2.54 cm}{1 in})^3 = 0.0361 lb / in3.


This can then be converted to lb/ft3:


0.0361 lb / in3   \frac{lb}{in^3} x (\frac{12 in}{1 ft})^3 = 62.4 lb / ft3.


It is important to notice that we have used conversion factors to convert from one unit to another unit of the same parameter