Answer is: the partial pressure of the hydrogen 500 mmHg.
p(total) = 523.8 mmHg.
p(water) = 23.8 mmHg.
p(total) = m(water) + p(hydrogen).
p(hydrogen) = p(total) - p(water)..
p(hydrogen) = 523.8 mmHg - 23.8 mmHg.
p(hydrogen) = 500 mmHg.
mmHg or millimeter of mercury is a manometric unit of pressure.