Why enthalpy of combustion is always negative ?

What is the mass of sodium in 3 moles of sodium chloride

Achange in the number of neutrons in an atom will change an blank . when the number of protons changes in an atom, a new element will form.

Aradio signal from a gps satellite take only about 0.067 seconds to reach a gps reciever. if the speed of light is about 300,000km/s, then approximately how far away is the reciever from from the satellite?