$C_\overline{n}H_{2\overline{n}-2}O_2\overset{+O_2}{\rightarrow}\overline{n}CO_2+(\overline{n}-1)H_2O $ $x \overline{n}x (\overline{n}-1)x$
$CO_2+Ca(OH)_2\rightarrow CaCO_3\downarrow+H_2O$
$\Rightarrow \frac{3,42\overline{n}}{14\overline{n}+30}=\frac{18}{100}\Rightarrow \overline{n}=6 \Rightarrow x=0,03 mol$
$\Rightarrow \Delta m=18-m_{CO_2}-m_{H_2O}=18-6.0,03.44-5.0,03.18=7,38$
$\Rightarrow $khối lượng dung dịch giảm $7,38 gam$