How does a physical change obey the law of conservation of mass?

What pressure will be exerted by 0.675 moles of a gas at 25*c if it is in a 0.750-l container?

The human eye contains a molecule called 11-cis-retinal that changes shape when struck with light of sufficient energy. the change in shape triggers a series of events that results in an electrical signal being sent to the brain that results in vision. the minimum energy required to change the conformation of 11-cis-retinal within the eye is about 164 kj/mol.

Calculate the energy required to ionize a hydrogen atom to an excited state where the electron is initially in the n = 5 energy level. report your answer in kilojoules