Atomic Mass:
The atomic mass of an element is the weighted average of the masses of its atoms.
The unit of atomic mass is called, fittingly enough, the atomic mass unit.
It is defined as one-twelfth the mass of the 12C atom, and it is abbreviated amu in most books but 'u' in some.
(A few chemists use the dalton as the unit of atomic mass, in honor of John Dalton.)
Although John Dalton in his atomic theory postulated that all atoms of the same element had the same mass, we now know that the different isotopes of an element have different masses.
For example, 12C atoms have a mass of exactly 12 amu each, whereas 13C atoms have a mass of 13.00335 amu each.
It turns out that the ratio of isotopes of each of the elements in all naturally occurring samples is very constant (to three or more significant digits), so the weighted average of the masses of the atoms of an element is constant, which is why Dalton’s hypotheses worked.
Please note that atomic mass, called atomic weight in some texts, is different from mass number.
The mass number refers to a specific isotope, and is an integer—the number of protons plus neutrons in each atom. The atomic mass is based on the naturally occurring mixture of isotopes, and is not an integer.
Atomic masses for almost all the elements are presented in the periodic table; mass numbers are presented there only for elements that do not occur naturally.
Note that atomic masses vary from about 1 for hydrogen to a little over 250 for the largest elements.
If we ever solve a problem and get an atomic mass outside this range, we know we have likely made a mistake.
Also note that no atom has an atomic mass in the range of 1 to 250 grams.
That might be the mass of a mole of atoms The weighted average of several sets of items is the average with regard to the number in each set.
For example, if the Jones family has triplet boys, each weighing 90 pounds, and one girl, who weighs 50 pounds, the average of one boy and the girl is 70 pounds, but the weighted average of all the children is 80 pounds:
90 pounds
90 pounds or 3(90 pounds) + 50 pounds = 320 pounds
90 pounds
50 pounds
-----------------
320 pounds
Weighted average = (320 pounds)/(4 children) = 80 pounds .
The atomic mass of each element can be determined in two different ways:
(1) as was done historically, by comparing the naturally occurring mixture versus a standard (now 12C), or
(2) as presently done with the modern mass spectrometer, by measuring the mass of each isotope and the percentage abundance of each.
Molar Mass or Molecular Mass
MOLAR MASS:
It is defined as the mass of one mole of given substance is known as molar mass. In other words also known as molecular weight.Because the molar mass is equal to the mass of one molecule of the given substance.
CALCULATION OF MOLAR MASS OR MOLECULAR WEIGHT:
Molar mass (or) Molecular weight = Sum of the Product of Number of atoms and atomic weight .
For example:
1) Molar mass of HCl:
atomic weight of Hydrgen = 1 g
atomic weight of Chlorine = 35.5 g
Here, in HCl there is "1" Hydrogen and "1"Chlorine.
Molar mass of HCl = (1 x Hydrgen atomic weight)+(1 x Chlorine atomic weight)
Molar mass of HCl = (1 x 1g)+(1 x 35.5g)
Molar mass of HCl = (1 g)+ (35.5 g)
Molar mass of HCl=36.5 g.
2) Molar Mass of H2SO4:
atomic weight of Hyrogen = 1 g
atomic weight of Sulphur = 32 g
atomic weight of Oxygen = 16 g
Here, H2SO4 is having "2" Hydrogens, "1" Sulphur, & "4" Oxygens.
Molar mass of H2SO4 = ( 2 x Hydrogen At.Wt.)+(1 x Sulphur At.Wt.)+(4 x Oxygen At.Wt)
Molar mass of H2SO4 = (2 x 1 g)+(1 x 32g)+(4 x 16 g)
Molar mass of H2SO4 = (2 g)+(32 g)+(64 g)
Molar mass of H2SO4 = 98 g.
3) Molar mass of NaOH:
Atomic weights of Sodium = 23 g.
atomic weight of Oxygen = 16 g.
atomic weight of Hydogen = 1 g.
Here, we have "1' Sodium,'1" Oxygen & "1" Hydragen atoms .
Molar mass of NaOH = (1 x Sodium At.Wt.)+(1 x Oxygen At.Wt)+(1 x Hydrogen At.Wt.)
Molar mass of NaOH = (1 x 23 g )+(1 x 16 g)+(1 x 1 g)
Molar mass of NaOH = (23 g)+(16 g)+(1 g)
Molar mass of NaOH = 40 g.
Atomic mass
Atomic mass :
The mass retained with an atom is known as atomic mass.And this is also known as the mass number of an atom.And also known as the average mass of the Naturally available abundant isotopes of the element expressed in amu (atomic mass unit).
Calculation of mass number:
Mass number = Number of Protons + Number of Neutrons.
Atomic weights of elements in Periodic Table:
Note:
The atomic weights which are expressed in the periodic table are the average mass of the naturally available abundant isotopes.
For example:
1) Atomic weight of chlorine:
Natural abundance of chlorine -35 is 75.77% = (35 x 75.77%) = 26.51amu.
Natural abundance of chlorine-37 is 24.23% = (37 x 25.23%) = 9.33amu.
Addition of the both will give the Atomic weight of the element.
= (26.51 + 9.33) amu.
= 35.84 amu.
Is equal to approximate weight, which is written in the periodic table.
EXAMPLE :2
(a) The atoms of a certain element have a mass 2.026 times the mass of an equal number of 12C atoms. What is the atomic mass of the element? (b) Which elementis it? (c) Whatis the bestway to make sure that we get equal numbers of atoms of two elements to compare total masses?
Solution
(a) The mass of the average atom is 2.026 times that of a 12C atom:
= 2.026(12.00 amu)
= 24.31amu.
(b) Magnesium (see the periodic table).
(c) The best way to get equal numbers of atoms is to make a compound of the two elements that has them in a 1 : 1 ratio.
EXAMPLE:3
Naturally occurring magnesium consists 78.70% of 24Mg, with atoms of mass 23.98504 amu, 10.13% of 25Mg, with atoms of mass 24.98584 amu, and 11.17% 26Mg, with atoms of mass 25.98259 amu. Calculate the atomic mass of magnesium
solution:
[(78.70%)(23.98504 amu) + (10.13%)(24.98584 amu) + (11.17%)(25.98259 amu)]/100.0%
= 24.31amu.
Atomic masses are used to describe combined as well as uncombined atoms.
Related Tags
Atomic Mass Table and molar mass , molar mass value in Atomic Mass Table