Molar Mass

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Molar Mass

Chemistry deals with the compositions of atoms and molecules. There are innumerable number of reactions where the atoms and molecules react with each other and form products. Molar mass of molecules and compounds play a very important role in different stoichiometric calculations. Every atoms or compound have both physical and chemical properties. Molar mass is one the physical property of the given substance.  It can also be referred as the mass of the given molecule or compound.

Difference between amu (Atomic mass Unit) and grams/mol.

The Atomic mass unit or the ‘amu’ is the units that is used for measuring the mass of each atom. Periodic table is often referred to find mass of the atoms. In the periodic table the mass of each atom is given in terms of amu. This atomic mass unit of an individual atom is taken to find the molar mass of a compound or a molecule. The unit of molar mass is however ‘grams/mol’ as it’s the combined mass of the atoms. Shown below are examples related to the calculation of the molar mass.
The Molar mass of one mole of CuSO4.
Given is one mole of Copper Sulfate CusSO4. Follow the steps below for finding the Molar mass:
·         One mole of CuSO4 consists of 1 Copper atom, 1 Sulphur atom and 4 atoms of oxygen.
·         Refer to the periodic table get the atomic mass number of Copper (Cu), Sulfur (S) and Oxygen (O) atoms.
·         For finding the molar mass of CuSO4 add the masses of the atoms as shown below:
Mass of CuSO4 = (Atomic mass of Copper) + (Atomic mass of Sulfur) + 4 (Atomic mass of Oxygen)
molar mass
The periodic table is always used to find the mass number of the given atoms. Cu has a mass number of 63.456 amu, Sulfur S has a mass number of 32.065 amu, and Oxygen has a molar mass of 15.9994 amu. Adding all the atomic mass units gives the following:
            Molar Mass = 1(63.456) + 1(32.065) + 4(15.9994)
                                 = 63.456 + 32.065 + 63.9976.
                                  = 159.6086 g/mol.
The Molar mass of 1 moles of NH3.
Given is 1 moles of NH3 which is also called as Ammonia. Steps for finding the Molar mass of Ammonia:
·         One mole of NH3 consists of 1 Nitrogen atom, and 3 atoms of Hydrogen.
·         Refer to the periodic table get the atomic mass number of Nitrogen (N), Hydrogen (H).
·         For finding the molar mass of NH3 add the masses of the atoms as shown below:
Mass of NH3 = (Atomic mass of Hydrogen) + 3 (Atomic mass of Hydrogen)
molar mass
The periodic table is always used obtain the atomic mass number of the given atoms. N has a mass number of 14.0067 amu, Hydrogen has a mass number of 1.00974 amu.
            Molar Mass = [1(14.0067) + 3(1.00974)] g/mol
                                     = [14.0067 + 3.02922] g/mol
                                      = 17.03592 g/mol.
Rounding the answer to two decimal places. Follow the steps below:
molar mass

 Relation between Molar Mass and Number of Moles:

The molar mass of a substance is independent of the number of moles. This is because molar mass of the substance weighs according to the composition of its atoms. The total mass of the substance however increases with the increase in the number of moles. The Moles and Molar mass of a substance are connected by the formula stated below:
molar mass
The units for the mass of a substance is grams and the units for the moles is mol. This also implies that the units for the molar mass of a given substance is grams/mol. Shown below are some solved examples to find the number of moles for different substances.
1)    Find the molar mass of a 3 mole of an element weighing 60 grams.
Mass of the substance given = 60 grams.
Number of moles = 3 moles.
Using the formula:
molar mass
Molar mass = 60 grams/3 mol = 20 gm/mol.
2)      In a lab experiment the chemist used 9 moles of Fe2O3. Find mass of Iron Oxide consumed if the mass was totally consumed.
For calculating the mass of Iron Oxide Fe2O3, its molar mass is required.
·         One mole of Fe2O3 consists of 2 atoms of Iron and three atoms of Oxygen.
·         Refer to the periodic table to obtain the atomic mass unit for Iron and Oxygen.
·         The atomic mass unit of Iron is 55.845 amu and the atomic mass unit of Oxygen is 15.9994 amu.
Adding the atomic mass units gives:
molar mass
Molar mass of Fe2O3 = 2 (55.845) + 3 (15.994) = 159.6882 g/mol 
= 159.69 g/mol (approximately.)
                        Using the formula:
molar mass                       
                        Mass of Fe2O3 = 159.69 x 9 = 1437.21 grams.
3)      Calculate how many number of moles of H2O molecules weight 36 grams.
Given is the mass of H2O molecule = 36 grams.
Now find the Molar mass of H2O in the following steps below:
·         The H2O molecule has two Hydrogen atoms and one Oxygen atom.
·         For finding the molar mass of one H2O molecule look for the atomic masses of Hydrogen and oxygen.
·         Hydrogen atom weighs 1.00794 amu and Oxygen weighs 15.9994 amu.
Molar Mass of H2O = [2(1.00794) + (15.9994)] g/mol
                                   = [2.01588 + 15.9994] g/mol
                                   = 18.01528 g/mol.
                                   = 18.0 (On approximation.)
Use the Molar mass formula mentioned above for finding the number of Moles:
molar mass
Substituting the values gives: Moles = 36/18.0 = 2.0 moles. 

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