a,
$n_C=n_{CO_2}=n_{CaCO_3\downarrow}=\dfrac{50}{100}=0,5(mol)$
$\Rightarrow n_{H_2O}=\dfrac{35,5-0,5.44}{18}=0,75(mol)$
$\Rightarrow n_H=2n_{H_2O}=1,5(mol)$
$\%m_C=\dfrac{0,5.12.100}{11,5}=52,17\%$
$\%m_H=\dfrac{1,5.100}{11,5}=13,04\%$
$\Rightarrow \%m_O=100-52,17-13,04=34,79\%$
b,
$n_O=\dfrac{11,5.34,79\%}{16}=0,25(mol)$
$n_C : n_H : n_O=0,5:1,5:0,25=2:6:1$
$\Rightarrow$ CTĐGN $C_2H_6O$
$\Rightarrow$ CTPT $C_2H_6O$