Complete combustion of 0.858 g of compound X, gives 2.63 g of CO₂ and 1.28 g of H₂O. The lowest molecular mass, X can have

molar mass of CO₂ ​ ​= 2.630 g or since the molar mass of CO₂​ ​ is 44 g/mol
we have 0.05976 moles of CO₂​ ​ since there is 1 mole. so C in CO₂​
we have 0.05976 moles of C or 0.718 g of C
mass of H₂​​O = 1.280 g or since the molar mass of water is 18 g/mol
we have 0.07105 moles of H₂​​O, since there are 2 moles of H in 1 moles of H₂​​O = 0.14210 moles of H or 0.143 g of H molar ratio C:H =0.05976 : 0.14210
mass of C+H = 0.861 g
or, after dividing by the smallest 0.05976
molar ratio of C:H= 1.000 : 2.378
multiply by 3 to get the whole number 3.000 : 7.134
empirical formula is C₃​​H₇​​

minimum molar mass is 43 g/mol