  Characterizing prestellar cores in star-forming regions is an important step
towards the validation of theoretical models of star formation. Thanks to their
sub-arcsecond resolution, ALMA observations can potentially provide samples of
prestellar cores up to distances of a few kpc, where regions of massive star
formation can be targeted. However, the extraction of real cores from
dust-continuum observations of turbulent star-forming clouds is affected by
complex projection effects. In this work, we study the problem of core
extraction both in the idealized case of column-density maps and in the more
realistic case of synthetic 1.3\,mm ALMA observations. The analysis is carried
out on 12 regions of high column density from our 250 pc simulation. We find
that derived core masses are highly unreliable, with only {\em a weak
correlation between the masses of cores selected in the synthetic ALMA maps and
those of the corresponding three-dimensional cores}. The fraction of real
three-dimensional cores detected in the synthetic maps increases monotonically
with mass and remains always below 50\%. Above $\sim 1\,M_{\odot}$, the core
mass function derived from the column-density maps is steeper than that of the
three-dimensional cores, while the core mass function from the synthetic ALMA
maps has a slope closer to that of the real three-dimensional cores. Because of
the mass uncertainties, proper guidance from realistic simulations is essential
if ALMA observations of protoclusters at kpc distances are to be used to test
star-formation models.
