  The purpose of this work is to describe the assumptions built into the X-ray
spectrum fitting software XSPEC for the calculation of element abundances and
emission measure of a plasma and to describe the effects when those assumptions
are not accurate. The ratio of electron density to hydrogen density in XSPEC is
fixed at a constant. The correct ratio can be calculated from the ionization
states of the elements. We show the constant value used in XSPEC is valid to
within 3.5% for a solar abundance plasma. For a plasma that deviates from solar
abundance, such as hydrogen-poor or heavy element rich plasmas as found in the
ejecta of supernova remnants, this ratio can smaller by factors of 0.1 to
0.001. The hydrogen emission measure, defined by integral of electron density
times hydrogen density over plasma volume, is derived from the norm in XSPEC,
but one needs to include the hydrogen abundance factor. For other elements, the
emission measures are the XSPEC values multiplied by the element abundance
factors. Using the correct electron-to-hydrogen ratio and emission measures, we
show the correct electron density is smaller by the square root of the correct
electron density ratio divided by the XSPEC value. Element densities and total
masses (for given distance and volume) are larger by the abundance factors
divided by the above square root. Because hydrogen-poor plasmas occur in the
ejecta of Type Ia supernova remnants, previously estimated element masses from
X-ray spectra are likely significantly underestimated.
