Your expression sets the incoming h2o molconc to twice the value of h2 molconc in the presence of a negative h2 molconc flux. This can run into problems, since there is no check on conservation before the value is set. These text widgets are intended for simple fluxes only. If you have conversion of h2 to h2o occurring, you should add a chemical reaction or source terms, so that things get linearized and accounted properly.
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