  We explore how the expulsion of gas from star-cluster forming cloud-cores due
to supernova explosions affects the shape of the initial cluster mass function,
that is, the mass function of star clusters when effects of gas expulsion are
over. We demonstrate that if the radii of cluster-forming gas cores are roughly
constant over the core mass range, as supported by observations, then more
massive cores undergo slower gas expulsion. Therefore, for a given star
formation efficiency, more massive cores retain a larger fraction of stars
after gas expulsion. The initial cluster mass function may thus differ from the
core mass function substantially, with the final shape depending on the star
formation efficiency. A mass-independent star formation efficiency of about 20
per cent turns a power-law core mass function into a bell-shaped initial
cluster mass function, while mass-independent efficiencies of order 40 per cent
preserve the shape of the core mass function.
