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We report column densities of molecular gas in the W5 star-forming region astraced with OH 18-cm emission in a grid survey using the Green Bank Telescope.OH appears to trace a greater column density than does CO in 8 out of 15 casescontaining OH emission detections; the two molecules trace the same columndensities for the other 7 cases. OH and CO trace a similar morphology ofmolecular gas with a nearly one-to-one correspondence. The mass of moleculargas traced by OH in the portion of the survey containing OH emission is $1.7$(+ 0.6 or - 0.2) $\times 10^4 M_{\odot}$, whereas the corresponding COdetections trace $9.9 \times 10^3 M_{\odot} (\pm 0.7) \times 10^3$. We findthat for lines observed in absorption, calculations assuming uniform gas andcontinuum distributions underestimate column density values by 1 to 2 orders ofmagnitude, making them unreliable for our purposes. Modeling of this behaviorin terms of OH cloud structure on a scale smaller than telescopic resolutionleads us to estimate that the filling factor of OH gas is a few to 10 percent.Consideration of filling factor effects also results in a method ofconstraining the excitation temperature values. The total molecular gas contentof W5 may be approximately two to three times what we report from directmeasurement, because we excluded absorption line detections from the massestimate.

OH as an Alternate Tracer for Molecular Gas: Quantity and Structure of Molecular Gas in W5