The molecular mass of benzene is 78 amu. Its percentage composition is 92.31 % C and 7.69 % H. Determine the molecular formula of benzene.

Answer

`C_(6)H_(6)`

Explanation

Step I. Empirical formula of benzene
`{:("Element","Percentage","Atomic mass","Gram atoms (Moles)","Atomic ratio (Molar ratio)","Simplest whole no. ratio"),("C",92.31,12,(93.31)/(12)=7.69,(7.69)/(7.69)=1,1),("H",7.69,1,(7.69)/(1)=7.69,(7.69)/(7.69)=1,1):}`
Empirical formula of benzene = CH
Step II. Molecular formula of benzene
Empirical formula mass `= 12 +1 =13u`
Molecular mass `= 78 u`
`n=("Molecular mass")/("Empirical formula mass")=((78u))/((13u))=6`
Molecular formula `= n xx` Empirical formula `=6xx CH = C_(6)H_(6)`.