how is mass number is calculated?

Answer 1

just add the number of protons and that of neutrons

Answer 2

In determining the elemental composition for an unknown compound from an exact mass measurement it helps to know as much as possible about the molecule being studied. Even if the molecule is a true unknown it is never truly unknown. We can easily ask, "Is it a protein?", "Is it a small molecule?", "If it is a small molecule what are the functional groups?" Use UV, IR, NMR, chemical modification, enzyme digest, or N-terminal protein sequencing to try and answer a few of these questions. A little bit of knowledge about the molecule can help you restrict the number of returns obtained from an elemental calculator. Determining elemental composition from an exact mass measurement for a molecule which is essentially entirely unknown becomes difficult. In this case for the purpose of elemental composition determination usually exact mass measurements are performed on molecules below 500 amu with a required mass accuracy at or better than 5 mmu. With increasing mass the number of elemental combinations increases exponentially. One way to combat the large number of possibilities is to reduce the size of the unknown portion of the molecule. As mentioned above if the unknown is a protein, an enzyme digest may help reduce the size of the unknown portion of the compound, also if functional groups can be identified these masses can be subtracted from the measured mass to help reduce the number of returns. It is difficult to determine the elemental composition of larger entirely unknown molecules. This is not to say that exact mass of larger molecules is useless. This information can be used in conjunction with other analytical analyses and in this case it becomes a valuable tool. Exact mass is especially useful when a large part of the molecule is known and a minor unknown change needs to be characterized.
Exact mass traditionally has been obtained by FAB magnetic-sector instruments and more recently by FTMS, MALDI TOF and quadrupole time of flight type instruments. Some scientists attempt to push their quadrupole mass spectrometers to obtain exact mass measurements. We have heard multiple reports of scientists using their quradrupole instruments to obtain exact mass, and while we believe their reports, in our experience we have been unwilling to claim better than 100 mmu accuracy for several of the most popular triple quadrupole instruments that we have worked with. As stated previously a mass spectrometer can never be too accurate or possess too much resolving power. As a side note, some higher end MALDI instruments are capable of routinely delivering mass accuracy for peptides to 40 mmu without an internal standard and to 5-10 mmu with an internal standard. In our experience the better MALDI instruments have beat the standard quadrupole or triple quadrupole instrument for consistent mass accuracy for peptide mass measurement, especially for peptides below 2000 amu.

Answer 3

Add the mass numbers of every atom in the compound.

Answer 4

The number of protons and neutrons

Answer 5

it is the sum of the number of protons and neutrons.

Answer 6

Wait until the priest shuts the doors and then do a headcount.