Determine the empirical and molecular formula:
ibufuren, a headache remedy contains 75.6% c, 8.08% h, and 15.5% o by mass and has a molar mass about 206 g/mol.

The empirical formula = molecular formula = C13H18O2

Explanation:

in 100% compound we have 75.6 % Carbon ( Molar mass = 12g/mole), 8.80% hydrogen ( Molar mass = 1.01 g/mole) and 15.5% Oxygen (Molar mass = 16.01 g/mole).

Carbon: 75.6g / 12 = 6.29

Hydrogen: 8.80/ 1 = 8.80

Oxygen: 15.5/ 16 = 0.97

⇒0.97 is the smallest so we divide everything through by 0.97

C: 6.29 / 0.97 =  6.48 ≈ 6.5

H: 8.80 /0.97 = 9

O: 0.97 / 0.97 = 1

To get rid of decimals, we multiply by 2  

C: 6.5 x 2 = 13

H: 9 x 2 = 18

O: 1 x 2 = 2

The empirical formula = C13H18O2

13x 12g/mol + 18x1g/mol  + 2x 16g/mol = 156 + 18 + 32 = 206g/mol  which is the molar mass of ibuprofen

The empirical formula = molecular formula = C13H18O2

"standard molar enthalpy:

h2o ( liquid water ) : - 285.8 kj/mole

h2o ( water vapor ) : - 241.8 kj / mole.

δhrxn = - 241. 8 - ( - 285.8 )= -241.8 + 285.8 = 44.0

answer: 44.0 kj"