Half life period

The half-life period for the thermal decomposition of phosphine at three different pressures are given below: {:("Pressure in mm. Hg",707 ,79 , 3.5 ),("Half-life period in seconds", 84,84,83):} Find the order of the reaction.

When inversion of sucrose is studied at pH = 5, the half-life period is always found to be 500 minutes irrespective of any initial concentration but when it is studied at pH = 6, the half-life period is found to be 50 minutes. Derive the rate law expression for the inversion of sucrose.