`n_(C_6H_12O_6)=300/180=5/3` (mol)

`C_6H_12O_6`$\xrightarrow{xt,t^o}$`2C_2H_5OH+2CO_2`

`n_(C_2H_5OH)=2n_(C_6H_12O_6)=2·5/3=10/3` (mol)

`m_(C_2H_5OH)=10/3·46=460/3 (g)`

`H=138/(460/3)·100%=90%`