In this universe ,whatever we measure,to measure we need a standard.To learn the property of gases we compare the quality/ quantity of gas with the  gas at S.T.P condition
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Volume of gas is change with changing tempratue and pressure. from boyle's low volume of gas is inversely praposnal to pressure.
V ∝ 1/V  
and from chales law volume of gas is directly praposnal to temprature. 
V ∝ T
and also temperature and air pressure vary from place to place it is necessary with a standard reference condition to compare with our testing for apropriate result. 
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