How can i get the value of ānā for an aclcohol such that the percentage by mass of oxygen equals 21.6%?

General formula of alkanol is CnH2n+1OH

So which means : CnH2n+2 O

If O is 21.6 % then remaining mass is due to C and H

Moles of carbon and H are in the ratio of

n:2n+2

Now use your algebra skill and mole concept of chemistry to solve it.