The molar mass (M) is a physical property in chemistry. It is defined as the mass of a given substance (chemical element or compound) divided by its amount of substance. The SI unit for molar mass is Kg/mol. For significant reasons, molar masses are always expressed in g/mol.
Molar mass of H2O: M (H2O) = 18g/mol
A mole can be defined as a collection of atoms whose mass is equal to the atomic mass in grams. The number of atoms in a mole is 6.02 x 1023. This number is known as Avogadro’s number.
1 mole H2O has the same number of molecules as in 18.015 g of H2O. Or one mole hydrogen molecule contains the same number of molecules as in 2.016 g of H2. The mass of one mole of a substance is known as Molar Mass.
Water is made up of hydrogen and oxygen, with a formula of H2O. There are 2 moles of Hydrogen and 1 moles of Oxygen. Therefore, Molar mass of water would be: 2 x H + O = 2 x 1.008 + 16 = 18.016 g/mol = 18 g/mol
The volume of the liquid usually can be calculated very easily. We use the gas equation
P V = n R T
P is the pressure of the gas
V is the volume of the gas
n is the amount of substance of gas (also known as number of moles)
T is the temperature of the gas
After calculating the number of moles of the gas, we can use the relationship:
Moles of the liquid = Mass in grams / Molar mass
So, Molar mass can be calculated as: Molar mass = Mass in grams / Moles of the liquid
This method of finding the molar mass of a liquid is called as VICTOR- MEYER’S METHOD.
There is a difference between molecular mass and molar mass. Molar mass is never measured directly. It is calculated by summing up the atomic masses. The molecular mass is measured directly. For example, molecule of water and molecule of heavy water. The molecular masses are different as both have different isotopes of hydrogen.
It is a very useful term and is used in different chemical calculations of stoichiometry and other colligative properties. For example, in the Avogadro’s gas law equation:
PV = nRT
Molar mass of a compound is usually calculated by adding the individual molar mass of all the elements present in it.
Example 1: How to calculate the molar mass of Sodium hydroxide?
The formula of Sodium hydroxide is NaOH. It contains sodium, hydrogen and oxygen.
Let us add the molar masses of all these elements:
Na: 22.98 g/mol
O: 16 g/mol
H: 1.008 g/mol.
Adding all the atomic masses:
22.98 + 16 + 1.008 = 39.98 g/mol
The molar mass of Sodium hydroxide is:
40gram /moles (rounded of)
Example 2: Calculate the molar mass of Copper sulphate.
Copper sulphate contains: Cu + S + 4 x O
Cu = 63.55 g/mol
S = 32 g/mol
O = 16g/mol
Cu + S + 4 x O = 63.55 + 32 + 4 x 16 = 159.55 = 160 g/mol.
So, molar mass of Copper sulphate is 160 grams/mol.
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