NCERT Solutions for class 11 science Molar Mass
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Any matter whether it is a chemical element or a chemical compound has some mass and occupies space. It is very easy to measure the masses of various substances (in kilograms/ grams) in our daily life, by using different kinds of weighing machines/balances. But how can we measure the mass of compounds or molecules? In the science laboratory, we use a tool called an analytical balance to measure in grams. In a chemical reaction, there are several reactants (elements/molecules/ compounds) that participate and give new products (different molecules/ compounds, from those in the reactants).But, how we can determine the amount/or masses of the reactants or products.
A Mole (abbreviated as a mole), gives the amount of a substance (1 mole contains 6.022×1023 entities). A Molar mass (M) also known as Molecular weight, of a substance is defined as mass per mole of the entities(atoms, molecules, or formula units) that make up the substance.
The periodic table is very important for calculating the molar mass.The periodic table(shown in the figure below) is a chart used in chemistry that shows the arrangement of the chemical elements in order of increasing atomic number. In a periodic table, the elements with similar atomic structure (hence similar chemical properties) appear in vertical columns.
The periodic table is a large grid of every element that exists. The elements are arranged in order of their atomic number. The atomic number is the number of protons that each atom has in its nucleus. By arranging the elements in this way, those with similar properties (characteristics) are grouped together. As with any grid, the periodic table has rows running left to right, and columns running up and down. The rows are called PERIODS and the columns are called GROUPS. Hydrogen (H) is the first element in the periodic table because it has just one proton in its nucleus. Helium (He) is second, because it has two protons, and so on. The periodic table can be colored-coded. Often, each group is given a particular color so that it is easy to pick out all the elements that belong to a particular group.
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Determining Molar Mass
- For Elements : To find out the molar mass of an element, we look up the atomic mass of the given element, and see whether the element is monoatomic or diatomic, for the monoatomic element; the molar mass is the same value of atomic mass that is given in the periodic table. For example, the molar mass of neon is 20.18 g/mol and molar mass of gold is 197.0 g/mol. In case of diatomic or molecular elements; we multiply the mass given in the periodic table with the number of atoms present. For example; the molar mass of oxygen molecule O2 ( oxygen occurs mostly as adiatomic molecule) is twice that of O. From periodic table (the atomic mass of O is 16 amu) the molar mass of one O is 16 g/mol. For two O atoms, the molar mass is 2×16=32 g/mol.
Molar mass (M) of O2 = 2x M of O = 2×16 g/mol = 32.00 g/mol
The most common form of sulphur exists as octatomic molecules, S8 ; so its molar mass is equal to:
Molar mass (M) of S8 = 8 x M of S = 8 x 32.07 = 256.6 g/mol
(From periodic table, atomic mass of one sulphur atom is 32.07 amu so molar mass of one sulphur atom is equal to 32.07 g/mol)
- For Compounds: For compounds, the molar mass is the sum of the molar masses of the atoms in the formula of that compound. Thus, from the formula of sulphurdi oxide, SO2 it is clear that there is 1 atom (which is equivalent to 1 mole) of sulphur and 2 atoms of oxygen (equivalent to 2 moles).
Molar mass of SO2 = Molar mass of S + (2 x molar mass of Oxygen)
Molar mass of SO2 = 32.07 g/mol + (2 x 16.00 g/mol)
Molar mass of SO2 = 64.07 g/mol
For ionic compound K2S, potassium sulphide;
Molar mass of K2S = (2 x Molar mass of K) + Molar mass of S
Molar mass of K2S = (2 x 39.10 g/mol) + 32.07 g/mol
Molar mass of K2S = 110.27 g/mol
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