Unit of Mass the Kilogram

The mass of an atom depends on the number of electrons, protons, and neutrons it contains and all atoms of a given isotope are identical in mass. The SI unit of mass (the kilogram) is too large to function as a convenient unit for the mass of an atom, thus a smaller unit is desirable. In 1961, the International Union of Pure and Applied Chemistry (lUPAC) defined the atomic mass unit (u) to be exactly equal to one-twelfth the mass of one carbon-12 atom. Carbon-12 ( C) is the carbon isotope that has six protons, six neutrons, and six electrons. Using this definition, we have that 1 u = 1.660539 X 10 kg. The atomic mass (sometimes called atomic weight) of an atom is then defined, relative to this standard, as the mass of the atom in atomic mass units (u). For example, the two naturally occurring isotopes of hefium, He and " He, have atomic masses of 3.01602931 u and 4.00260324 u, respectively. This means that a helium-4 (" He) atom is 4.00260324/12 = 0.33355027 times as massive as a carbon-12 atom. ... 

Since the first CGPM in 1889, the unit of mass, the kilogram, has been defined by an international prototype, a metal block made of a platinum-iridium alloy, kept at the BIPM at Sevres. The relevant declaration was modified slightly at the third CGPM in 1901 to confirm that The kilogram is the unit of mass it is equal to the mass of the international prototype of the kilogram. 

The mass of an object is a measure of the quantity of material it contains. The SI unit of mass, the kilogram (kg), is used for larger masses, such as body mass. The standard for mass, the international prototype kilogram (IPK), is a cylinder that is made of a platinum-iridium alloy. In the metric system, the unit for mass is the gram (g), which is used for smaller masses. There are 1000 g in one kilogram. In comparison to the U.S. system, the mass of 1 kg is equivalent to 2.205 lb, and 453.6 g is equal to one pound. Some useful relationships between different units for mass follow ... 

In this last chapter the technique of thermogravimetry will be treated, by no means the least of the thermal analysis techniques. The principles of the technique are illustrated in Fig. 7.1 in the upper right-hand comer. The sample, indicated by number 3, is kept in a controlled furnace, 2, whose temperature is monitored by the thermocouple, 4, via the millivoltmeter, 5. The balance, 1, allows continuous mass determination. The present-day unit of mass, the kilogram, has been described in Fig. 2.17. A plot of mass as a function of temperature, T, or time, t, represents the essential thermogravimetry result. 

According to the modern convention, measurable quantities are expressed in SI (System Internationale) units and replace the centimetre-gram-second (cgs) system. In this system, the unit of length is a metre (m, the unit of mass is kilogram (kg) and the unit of time is second (s). All the other units are derived from these fundamental units. The unit of thermal energy, calorie, is replaced by joule (1 J = 107 erg) to rationalize the definition of thermal energy. Thus, Planck s constant... 

To express quantities much larger or smaller than the standard units, multiples or submultiples of these units are used, as shown in the Table 1-1. Thus, lO12 s is a picosecond (ps), and 103 m is a kilometer (km). Since for historical reasons the SI reference unit for mass, the kilogram, already has a prefix, multiples for mass should be derived by applying the multiplier to the unit gram rather than to the kilogram. Thus lO9 kg is a microgram (KT6 g), abbreviated pg. 

The CIPM has approved twenty prefixes for SI units (see Table 3) and permits the use of any SI prefix with an SI unit, with one exception. The SI unit for mass, the kilogram, already has a prefix in its name and can have no other SI prefix. To use prefixes with a unit for mass, the rule is to remove the kilo prefix and add the new prefix to gram (unit symbol g), as in milligram and its abbreviation mg. 

Because atoms are so tiny, the normal units of mass—the gram and the kilogram—are much too large to be convenient. For example, the mass of a single carbon atom is 1.99 x 10 g. To avoid using terms like 10 when describing the mass of an atom, scientists have defined a much smaller unit of mass called the atomic mass unit, which is abbreviated amu. In terms of grams,... 

Table 2.1 lists tiie standard imits in the SI system. They include the meter (m) as the standard unit of lengtii the kilogram (kg) as the standard unit of mass and the second (s) as the standard unit of time. Each of tiiese standard units is precisely defined. The meter is defined as fhe distance light travels in a certain period of... 

When making measurements, we must be consistent in our use of units. Numbers should always be written with their corresponding units, and units guide our way through calculations. The standard SI unit of length is the meter (m) of mass, the kilogram (kg) and of time, the second (s) (2.4). 

Table 1.1 shows the standard SI base units. For now, we focus on the first four of these units the meter, the standard unit of length the kilogram, the standard unit of mass the second, the standard unit of time and the kelvin, the standard unit of temperature. 

There are a few basic numerical and experimental tools with which you must be familiar. Fundamental measurements in analytical chemistry, such as mass and volume, use base SI units, such as the kilogram (kg) and the liter (L). Other units, such as power, are defined in terms of these base units. When reporting measurements, we must be careful to include only those digits that are significant and to maintain the uncertainty implied by these significant figures when transforming measurements into results. 

Mass. The unit of mass is the kilogram and is the mass of a particular cylinder of Pt—Ir alloy which is preserved in France by the International Bureau of Weights and Measures. 

Kilogram. The kilogram is the unit of mass it is equal to the mass of the international prototype of the kilogram. 

Inasmuch as only mass ratios are involved in these calculations, kilograms or any other unit of mass maybe substituted for pounds without affecting the validity of the example. 

Two systems of units are in common usage in mechanics. The first, the SI system, is an absolute system based on the fundamental quantities of space, time, and mass. All other quantities, including force, are derived. In the SI system the basic unit of mass is the kilogram (kg), the basic unit of length (space) is the meter (m), and the basic unit of time is tbe second (s). The derived unit of force is the Newton (N), which is defined as the force required to accelerate a mass of 1 kg at a rate of 1 m/s-. 

For scientific work the fundamental standard of mass is the international prototype kilogram, which is a mass of platinum-iridium alloy made in 1887 and deposited in the International Bureau of Weights and Measures near Paris. Authentic copies of the standard are kept by the appropriate responsible authorities in the various countries of the world these copies are employed for the comparison of secondary standards, which are used in the calibration of weights for scientific work. The unit of mass that is almost universally employed in laboratory work, however, is the gram, which may be defined as the one-thousandth part of the mass of the international prototype kilogram. 

These prefixes should be used with great care and be written immediately adjacent to the unit to be qualified furthermore only one prefix should be used at a time to precede a given unit. Thus, for example, 10 3 metre, which is one millimetre, is written 1 mm. 103 kg is written as 1 Mg, not as 1 kkg. This shows immediately that the name kilogram is an unsuitable one for the basic unit of mass and a new name may well be given to it in the future. 

---