The answer is #693.5 mmHg#.
The total pressure reqistered for the collected gas represents the sum of the partial pressures of water(its vapor pressure ) and hydrogen gas. We know that
#P_(TOTAL) = P_(H_2) + P_(water)#
Usually, water's vapor pressure at various temperatures is given, but you could try and calculate it using the given temperature by
#P = exp(20.386 - 5132/T) mmHg#, where temperature is in K and pressure in mmHg. Using this approximation formula (wikipedia link: http://en.wikipedia.org/wiki/Vapour_pressure_of_water) we get
#P_(water) = exp(20.386 - 5132/(273.15+30)) = 31.7 mmHg#
I've looked up the actual value of the vapor pressure at that temperature and it's listed at
#P_(water) = 31.5 mmHg#, which is pretty close to the value determined using the approximation formula.
Let's use the #31.5 mmHg# vapor pressure. This will result in
#P_(H_2) = P_(TOTAL) - P_(water) = 725 - 31.5 = 693.5 mmHg#
Water's vapor pressure values: