The sum of the percentage of carbon, hydrogen, and chlorine is not $100 \%$
Hence the rest of the part of the compound is oxygen
percentage of oxygen $=100-(18.6+1.55+55.04)=24.81 \%$
Simplest ratio $=1: 1: 1: 1$
Hence empirical formula $=$ CHCIO
Empiraical formula weight $=12 \times 1+1 \times 1+35.5 \times 1+16 \times 1=64.5$
$\frac{\text { molecular mass }}{\text { empirical mass }}=\frac{129}{64.5}=2$
Therefore molecular formula $=\mathrm{n} \times($ Empiricalformula $)$
Molecular formula $=2 \times(\mathrm{CHCIO})$
Molecular formula $\mathrm{C}_{2} \mathrm{H}_{2} \mathrm{Cl}_{2} \mathrm{O}_{2}$