Question

How can we find the atomic weight or mass of any element ?

Answer 1

WHAT IS ATOMIC MASS?
Atomic mass is the sum of the masses of the protons, neutrons, and electrons in an atom, or the average mass, in a group of atoms. However, electrons have so much less mass than protons and neutrons that they don't factor into the calculation. So, the atomic mass is the sum of the masses of protons and neutrons. There are three ways to find atomic mass, depending on your situation.
Which one to use depends on whether you have a single atom, a natural sample of the element, or simply need to know the standard value.
3 WAYS TO FIND ATOMIC MASS
1) LOOK UP ATOMIC MASS ON THE PERIODIC TABLE
If it's your first encounter with chemistry, your instructor will want you to learn how to use the periodic table to find the atomic mass (atomic weight) of an element. This number usually is given below an element's symbol. Look for the decimal number, which is a weighted average of the atomic masses of all the natural isotopes of an element.
Example: If you are asked to give the atomic mass of carbon, you first need to know its element symbol, C. Look for C on the periodic table. One number is carbon's element number or atomic number. Atomic number increase as you go across the table. This is not the value you want. The atomic mass or atomic weight is the decimal number, The number of significant figures varies according to the table, but the value is around 12.01.
This value on a periodic table is given in atomic mass units or amu, but for chemistry calculations, you usually write atomic mass in terms of grams per mole or g/mol. The atomic mass of carbon would be 12.01 grams per mole of carbon atoms.
2) SUM OF PROTONS AND NEUTRONS FOR A SINGLE ATOM
To calculate the atomic mass of a single atom of an element, add up the mass of protons and neutrons.
Example: Find the atomic mass of an isotope of carbon that has 7 neutrons. You can see from the periodic table that carbon has an atomic number of 6, which is its number of protons. The atomic mass of the atom is the mass of the protons plus the mass of the neutrons, 6 + 7, or 13

Molar massis a unit that enables scientists to calculate the weight of any chemical substance, be it an element or a compound. Molar mass is the sum of all of the atomic masses in a formula. Once one determines the molar mass of a substance, it will be easy to measure out one mole of that substance.
The molar mass calculation of a substance is complete the following steps (We will use sulfuric acid, H2SO4, as an example):
Make a list of each element and the number of atoms of each element present in the substance.
H 2
S 1
O 4
Go to periodic table and determine the atomic mass average (atomic weight) of each element.
H 1.00794
S 32.066
O 15.9994
Multiply each atomic mass by the number of atoms in the formula.
H 1.00794 * 2 = 2.015
S 32.066 * 1 = 32.066
O 15.9994 * 4 = 63.998
Add up the results of step three:
2.015+32.066+63.998 = 98.079 = molar mass of sulfuric acid
In this example the results have been rounded off to the correct number of decimal places. (Since the atomic mass average of sulfur given above only has 3 decimal places, accuracy can not be determined beyond that point).