Yes. If the masses are the same, then it can be considered as a constant and thus neglected.
Not many things have no mass however Photons (Light Particles) have no mass and therefore they travel at the speed of light.
Proton: 1,007 276 466 77(10) atomic mass units (u) Electron: 5,485 799 094 3(23).10-4 atomic mass units (u) Mass ratio proton/electron: 1 836
The atomic mass number is the mass of the isotope of an element (the sum of number of protons and number of neutrons).
100g?