in SI system there is one unit for one physical quantity so system is called ?
Answers
The SI unit of quantity or amount of substance is the mole, a fundamental unit of the SI. There are no other modern units in which amount of substance is measured, so no conversion factors are required.
The Systeme International Metric
The International System of Units
Quantitative measurement is the cornerstone of modern science, but it has not always been so. The application of quantitative measurements to chemistry, for example, does not predate AD 1500. Quantitative measurement was developed for other purposes, as technology, and was only then adopted for scientific use. The system of weights and measures were developed on an ad hoc basis in different parts of the world. The most fundamental quantities measured were mass or weight, length or distance, and time. Systems of units for measuring these were developed from the very beginning of recorded history. Measurement of temperature was added in the sixteenth century, and measurement of electric current in the eighteenth century. More recently the amount of substance and luminous intensity have been added in the International System of Units, or SI.
The International System of Units or Systeme Internationale (SI) is an improved metric system adopted by the Eleventh General Conference of Weights and Measures in 1960. It is the universal measuring system used in all areas of science throughout the world. The entire SI system of measurement is constructed from seven base units, each of which represents a single physical quantity as shown in the table below.
Like earlier versions of the metric system, the SI units can be designated as decimal fractions or multiples by the use of appropriate prefixes. The acceptable SI prefixes are given in the table below.
Any prefix can be applied to any base unit except the kilogram; the kilogram takes prefixes as if the base unit were the gram. As a consequence 10-6 kg is written as 1 milligram (mg) rather than 1 microkilogram (ukg). Luminous intensity is rarely used in chemistry and we will not consider it further in this course, but the remaining six base units are essential to chemical studies.
The great advantage of the SI over other systems of units is that when any physical quantity whatever is written out in the SI base units or in units derived only from the SI base units, any mathematical manipulations performed with them will follow as well. No conversion factors will ever be required. This means that if the symbols in any equation are replaced by real numbers with their SI base units and algebraic manipulations are performed upon the units in exactly the same way as they are performed upon the numbers to which those units refer, the result will come out with the correct numbers and units.
Example. The mass of a sample of pure rhombic sulfur was 150.637 g and the volume of water it displaced was 72.8 mL. The density of sulfur is then (150.637 g)/(72.8 mL) = 2.07 g/mL, or g/cm3, or kg/dm3, or kg/L. This is 2.07(0.001 kg/g)(106cm3/m3) = 2.07 x 103 kg/m3.
Using the mass as 0.150637 kg and the volume as 72.8 x 10-6 m3. The density of sulfur is then (0.150637 kg)/(72.8 x 10-6 m3) = 2.07 x 103 kg/m3. The quantity calculated gives the result in SI base units without conversion. Reporting the answer as 2070 kg/m3, while arithmetically correct and in SI base units, would give the answer to one more significant figure than is justifiable from the measured data.
Base Units of the SI
Length
The SI unit of length is the metre, a fundamental unit of the SI. The metre was once defined in terms of the circumference of the earth as part of the older metric system. Since 1983 the metre is by definition the length of the path travelled by light in vacuum in 1/299792458 of a second. The micron (u) is an obsolete name for the micrometre (um). Conversion factors between other units of length and the metre are:
1 Angstrom = 10.0 nm (exactly)
1 inch = 25.4 mm (exactly); 1 foot = 0.3048 m (exactly); 1 yard = 0.9144 m (exactly); 1 mile = 1.609344 km (exactly)
1 astronomical unit (A.U.) = 149.51 ñ 0.05 Gm
Mass
The SI unit of mass is the kilogram, a fundamental unit of the SI. The kilogram was once defined as the mass of one cubic decimetre of water. Since 1901 it is by definition the mass of the international prototype of the kilogram, a platinum-iridium mass which is stored at Sevres in France. The metric tonne is a common name for the megagram (Mg). Conversion factors between other units of mass and the kilogram, or its subdivision the gram, are:
1 unified atomic mass unit (u) = 1.66... yg
1 pound (lb) = 453.59237 g (exactly); 1 ton (short, 2000 lb) = 907.18474 kg (exactly); 1 ounce = 1/16 lb