  We study the effect that non-equilibrium chemistry in dynamical models of
collapsing molecular cloud cores has on measurements of the magnetic field in
these cores, the degree of ionization, and the mean molecular weight of ions.
We find that OH and CN, usually used in Zeeman observations of the
line-of-sight magnetic field, have an abundance that decreases toward the
center of the core much faster than the density increases. As a result, Zeeman
observations tend to sample the outer layers of the core and consistently
underestimate the core magnetic field. The degree of ionization follows a
complicated dependence on the number density at central densities up to 10^5
cm^{-3} for magnetic models and 10^6 cm^{-3} in non-magnetic models. At higher
central densities the scaling approaches a power-law with a slope of -0.6 and a
normalization which depends on the cosmic-ray ionization rate {\zeta} and the
temperature T as ({\zeta}T)^1/2. The mean molecular weight of ions is
systematically lower than the usually assumed value of 20 - 30, and, at high
densities, approaches a value of 3 due to the asymptotic dominance of the H3+
ion. This significantly lower value implies that ambipolar diffusion operates
faster.
