## Measurements in Chemistry

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# Experiments & Measurements - 02

## Measurements in Chemistry

### Units of measure

Chemistry, like other sciences, uses the SI system of measurements, which has been covered in the monograph: System of Units .

#### Fundamental quantities

##### Mass

Note: Mass is a physical property that measures the amount of matter in a body, whereas weight measures the pull of gravity on the body. While the weight of a body depends on its location, the mass of the body is independent of its location. In chemistry, the terminology "weighing" really means measurement of mass by comparing two weights.

Mass is measured in SI units by the kilogram (kg). However, since the kilogram is too large for practical purposes in chemistry, other multiples of this unit are generally used. These are:

• the metric gram (g). [1g = 10-3kg]
• the milligram (mg). [1mg = 10-6kg]
• the microgram (µg). [1µg = 10-9kg]

In the laboratory, mass of a sample of matter is determined by weighing it against standard masses on a balance. Some common balances used in the lab are: Fig 1: A triple-beam balance. (Source: Whitten et al 2004, Chemistry, p. 18)
• Triple-beam balance: This is a mechanical balance normally found in school laboratories, and has a precision of about $± 0.01 g$.
• Top-loading balance: This is an electronic balance, and has a precision of about $± 0.001 g$.
##### Length

The meter (m) is the standard SI unit of length. Just as in the case of the kilogram, the meter is much too large for measurements in chemistry. The other commonly used units for length are:

• the centimeter (cm). [1cm = 10-2m]
• the millimeter (mm). [1mm = 10-3m]
• the micrometer (µm). [1µm = 10-6m]
• the nanometer (nm). [1nm = 10-9m]
• the picometer (pm). [1pm = 10-12m]
##### Temperature Fig 2: Comparison of the Temperature Scales. (Source: electrical-res.com 2010, celsius)

Over the years, the celsius or centigrade degree (°C) has been replacing the usage of the fahrenheit degree (°F). In scientific circles, however, the use of the kelvin (K) has become the norm. (Note that we say "kelvin", not "kelvin degree".)

Each unit of the kelvin and the celsius degree are the same, which is one-hundredth of the interval between the freezing and boiling points of water at standard atmospheric pressure. The only difference is that whereas the Celsius scale assigns a value of 0°C to the freezing point of water and 100°C to the boiling point, the Kelvin scale assigns a value of 0K to the coldest possible temperature, which is 273.15°C, sometimes called absolute zero. (Fig. ) Thus,

• 0K = 273.15°C, and
• 273.15K = 0°C.

In contrast, the Fahrenheit scale specifies 32°F as the freezing point of water, and 212°F as its boiling point, thus effectively creating and interval of 180° between the two. (Fig. )

To convert from celsius to fahrenheit, use:

$F = 9 5 C + 32$

.......(1)

where: C = temperature in celsius, F = temperature in fahreheit.

To convert from fahrenheit to celsius, use

$C = 5 9 ( F − 32 )$

.......(2)

##### Mole

Calculations in chemistry revolve around the mole, a fundamental SI unit which measures a physical quantity referred to as the "amount of substance."

An understanding of the mole requires pre-knowledge of some fundamental concepts in chemistry related to atoms and molecules, and so will be dealt with later on.

#### Derived quantities

##### Volume Fig 3: Measuring cylinder for volume measurements. (Source: Myers, Oldham & Tocci 2006, Holt Chemistry, p. 12)

The volume of an object is the amount of space occupied by it. The SI unit of volume is cubic meter (m3). However, a cubic meter is too large a quantity for practical use in chemistry. Therefore, the convenient units employed are:

• the cubic decimeter (dm3), which is equal to the metric liter (L). [1dm3 = 1L ]
• the cubic centimeter (cm3), which is equal to the metric milliliter (mL). [1cm3 = 1mL ]

The following conversions should be known by heart:

$1 cm 3 = 1 mL 1 dm 3 = 1 L = 1000 mL = 1000 cm 3 1 m 3 = 1000 L$

.......(3)

In the laboratory, apparatus like the volumetric flask, the burette and the graduated cylinder are used for measuring volumes of liquids.

##### Density

Density is the mass of an object divided by its volume. The SI unit of density is kg/m3, but for convenience in chemistry, density for solids is expressed in g/cm3, and for liquids in g/mL.

$density = mass volume$

.......(4)

### Accuracy, Precision and Significant Figures

The subject of significant figures, accuracy and precision in measurements has been covered in the monograph: Significant Figures .

# List of References

electrical-res.com, the celsius temperature scale, viewed 1 January, 2010, <http://www.electrical-res.com/the-celsius-temperature-scale/>, 2009.
Myers, RT, Oldham, KB & Tocci, S, Holt Chemistry, USA:Holt, Rinehart and Winston, 2006.
Whitten, KW, Davis, RE, Peck, L & Stanley, GG, General Chemistry, 7th edn, Belmont, USA: Thomson Brooks/Cole, 2004.

# Bibliography

McMurray, J & Fay, RC, Chemistry, 4th edn, USA: Prentice Hall, 2003.