  We create mock X-ray observations of hot gas in galaxy clusters with a new
extension of L-Galaxies semi-analytic model of galaxy formation, which includes
the radial distribution of hot gas in each halo. Based on the model outputs, we
first build some mock light cones, then generate mock spectra with SOXS package
and derive the mock images in the light cones. Using the mock data, we simulate
the mock X-ray spectra for ROSAT all-sky survey, and compare the mock spectra
with the observational results. Then, we consider the design parameters of HUBS
mission and simulate the observation of the halo hot gas for HUBS as an
important application of our mock work. We find: (1) Our mock data match the
observations by current X-ray telescopes. (2) The survey of hot baryons in
resolved clusters by HUBS is effective below redshift 0.5, and the observations
of the emission lines in point-like sources at z>0.5 by HUBS help us understand
the hot baryons in the early universe. (3) By taking the advantage of the large
simulation box and flexibility in semi-analytic models, our mock X-ray
observations provide the opportunity to make target selection and observation
strategies for forthcoming X-ray facilities.
