What do you want to convert?

you have stated that in a photocatalytic reactor of 150 ml (geometric) volume containing 100 ml water, after exposing the photocatalyst ( qty not specified) to radiation ( not defined wrt wavelength and intensity in Watts and the unspecified area of aperture) for a period of time ( not specified) a sample collected showed 100 ppm of hydrogen.

Now based on such insufficient information what are you trying to convert this into?

Ms Srinivasan I want to convert 100 ppm of hydrogen in to micromole ?

You may consider 100 ppm as volume ratio of hydrogen in the gas mixture. If we assume most gases occupy 22.4 lit per gmol, then 100 ppm is (100*10 power minus 6/22.4) will be gmol of hydrogen in the mixture. In terms of micromol, we can multiply by 10 power 6. So overall 100 ppm will be 4.5 micromol