When solid SnO₂ is added to an aqueous solution of NaOH, the

$2{\mathrm{Na}}^{+}\left(\mathrm{aq}\right)+2{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)+{\mathrm{SnO}}_2\left(\mathrm{s}\right)\to 2{\mathrm{Na}}^{+}\left(\mathrm{aq}\right)+{\mathrm{SnO}}_3^{2-}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}$
The number of ions decreases in the ratio of 4 to 3, and so also the colligative property.