In fifth group, ${\left({\mathrm{NH}}_4\right)}_2{\mathrm{CO}}_3$ is added to precipitate out the carbonates. We do not add ${\mathrm{Na}}_2{\mathrm{CO}}_3$ because:

${\mathrm{CaCO}}_3$ is soluble in ${\mathrm{Na}}_2{\mathrm{CO}}_3$

${\mathrm{Na}}_2{\mathrm{CO}}_3$ increases the solubility of fifth group carbonates

${\mathrm{MgCO}}_3$ will be precipitated out in fifth group

none of the above