**Category : **JEE Main & Advanced

The chosen standard of measurement of a quantity which has essentially the same nature as that of the quantity is called the unit of the quantity. Following are the important types of system for unit,

(1)** C.G.S. System :** Length (centimetre), Mass (gram), Time (second)

(2) **M.K.S. System :** Length (metre), Mass (kilogram), Time (second)

(3)** F.P.S. System :** Length (foot), Mass (pound), Time (second)

(4) **S.I. System :** The 11th general conference of weights and measures (October 1960) adopted International system of units, popularly known as the SI units. The SI has seven basic units from which all other units are derived called derived units. The standard prefixes which helps to reduce the basic units are now widely used. Dimensional analysis : The seven basic quantities lead to a number of derived quantities such as pressure, volume, force, density, speed etc. The units for such quantities can be obtained by defining the derived quantity in terms of the base quantities using the base units. For example, speed (velocity) is expressed in distance/time. So the unit is \[m/s\] or \[m{{s}^{-1}}\]. The unit of force \[\text{(mass}\times \text{ acceleration)}\] is \[kg\ m{{s}^{-2}}\] and the unit for acceleration is \[m{{s}^{-2}}\].

**Seven basic S.I. units**

Length | Mass | Time | Temperature | Electric Current | Luminous Intensity | Amount of substance |

metre (m) | Kilogram (kg) | Second (s) | Kelvin (K) | Ampere (A) | Candela (Cd) | Mole (mol). |

**Derived Units**

Physical quantity |
Unit |
Symbol |

Area | square metre | m2 |

Volume | cubic metre | m3 |

Velocity | metre per second | ms?1 |

Acceleration | metre per second square | ms?2 |

Density | kilogram per cubic metre | kg m?3 |

Molar mass | kilogram per mole | kg mol?1 |

Molar volume | cubic metre per mole | m3 mol?1 |

Molar concentration | mole per cubic metre | mol m?3 |

Force | newton (N) | kg m s?2 |

Pressure | pascal (Pa) | N m?2 |

Energy work | joule (J) | kg m2 s?2, Nm. |

**Standard prefixes use to reduce the basic units**

Multiple |
Prefix |
Symbol |
Submultiple |
Prefix |
Symbol |

1024 | yotta | Y | 10?1 | deci | d |

1021 | zetta | Z | 10?2 | centi | c |

1018 | exa | E | 10?3 | milli | m |

1015 | peta | P | 10?6 | micro | m |

1012 | tera | T | 10?9 | nano | n |

109 | giga | G | 10?12 | pico | p |

106 | mega | M | 10?15 | femto | f |

103 | kilo | k | 10?18 | atto | a |

102 | hecto | h | 10?21 | zeto | z |

101 | deca | da | 10?24 | yocto | y. |

** **

**Conversion factors**

1 m = 39.37 inch | 1 cal = 4.184 J | 1 e.s.u. = 3.3356 ´ 10?10 C | 1 mole of a gas = 22.4 L at STP |

1 inch = 2.54 cm | 1 eV = 1.602 ´ 10?19 J | 1 dyne = 10?5 N | 1 mole a substance = N0 molecules |

1 litre = 1000 mL | 1 eV/atom =96.5 kJ mol?1 | 1 atm = 101325 Pa | 1 g atom = N0 atoms |

1 gallon (US) = 3.79 L | 1 amu = 931.5016 MeV | 1 bar = 1 ´ 105 N m?2 | t (oF) = 9/5 t (oC) + 32 |

1 lb = 453.59237 g | 1 kilo watt hour = 3600 kJ | 1 litre atm = 101.3 J | 1 g cm?3 = 1000 kg m?3 |

1 newton =1 kg m s?2 | 1 horse power = 746 watt | 1 year = 3.1536 ´ 107 s | 1Å = 10?10 m |

1 J = 1 Nm =1 kg m2 s?2 | 1 joule = 107 erg | 1 debye (D) = 1 ´ 10 ?18 esu cm | 1nm = 10?9 m. |

*play_arrow*Significant figures*play_arrow*The mole concept*play_arrow*Units for measurement*play_arrow*Laws of chemical combination*play_arrow*Important hypothesis*play_arrow*Atomic, Molecular and Equivalent masses*play_arrow*Percentage composition & Molecular formula*play_arrow*Chemical stoichiometry*play_arrow*Limiting reagent or reactant

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