Answered By Mingrui (Mark) J

use the Ideal gas equation:  $P\cdot V=T\cdot n\cdot R$P·V=T·n·R ;

where P, V, T, n, R are the Pressure, Volume, Temperature (in Kalvin scale), mole number of the gas and Gas Constant.

First, change the temperature and mass into Kavin scale and mole number

   $T=25^{\circ}C=\left(25+273.15\right)K=298.15K$T=25^?C=(25+273.15)K=298.15K   

  $N=350g\cdot H\cdot\frac{1mol}{4g\cdot H}=87.5mol$N=350g·H·1mol4g·H =87.5mol 

put them in the equation:

 $P\cdot V=T\cdot n\cdot R$P·V=T·n·R 

 $P\cdot10L=87.5mol\cdot298.15K\cdot8314.46L\cdot Pa\cdot K^{-1}\cdot mol^{-1}=216908672\frac{mol\cdot K\cdot L\cdot Pa}{K\cdot mol}=216908672\cdot L\cdot Pa$P·10L=87.5mol·298.15K·8314.46L·Pa·K⁻¹·mol⁻¹=216908672mol·K·L·PaK·mol =216908672·L·Pa 

 $P=\frac{216908672\cdot L\cdot Pa}{10\cdot L}=21690867.2Pa=2.169\cdot10^7Pa$P=216908672·L·Pa10·L =21690867.2Pa=‍2.169·10⁷Pa 

 so we come to the answer here:

 $P=2.169\cdot10^7Pa$P=2.169·10⁷Pa