First you have to define your conditions. You cannot saturate with isobutane unless isobutane is a liquid under the conditions of interest. Here is a link to a vapor pressure graph for isobutane (
If your conditions are below the curve or above the critical temperature of isobutane there can be no liquid phase present and ammonia and isobutane will be completely miscible at all concentrations. This assumes that you are under conditions that ammonia is not a liquid. Here is a link to a vapor pressure graph for ammonia (
For ammonia to be a gas your conditions must be below the curve or above the critical temperature of ammonia. Assuming you are under conditions where ammonia is a vapor and isobutane can be a liquid, read off the vapor pressure for isobutane to get the partial pressure of isobutene. Subtract this from the total pressure to get the vapor pressure of ammonia. Assuming the ideal gas law and that ammonia is negligibly soluble in isobutane (both questionable assumptions), the ratio of moles of isobutane to moles of ammonia is equal to the partial pressure ratio. From the mole ratio you can calculate the mass ratio. If you need a better number you will need measured solubility of ammonia in isobutane and use real gas models for both. Your easiest option would be to enlist the aid of someone who has access to Aspen Plus (and knows what they are doing) and have them do your calculation.
