The chemical compound sodium sulfide has the formula Na2S. It is written as
. It has a crystalline state. Na is followed by a two or 2. this indicates that the molecule has two of the specified element's atoms.
There is just one of these because there is no number following the S.
In all, this molecule consists of one sulfur atom and two sodium atoms.
A compound's chemical formula, which uses symbols for combining atoms, reveals the composition of the complex. We require the valency of the combining parts in order to write the formula.
The sodium sulfide chemical formula must be written down. So sodium and sulfur make up the two elements in the chemical. Sodium has a valency of +1, while sulfur has a +2.
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At the center of every atom lies a small, dense "nucleus" that is positively charged.
Atom is the smallest unit of matter. It is made up of electrons, protons and neutrons. Electrons are negatively charged particles that are found scattered in different energy levels outside the nucleus. Protons are positively charged particles present inside the nucleus. Neutrons do not carry charge and found inside the nucleus. Thus, protons and neutrons make up the nucleus of an atom. Since, protons are positively charged and neutrons are uncharged particles, the charge of nucleus becomes positive.
Answer: Option (d) is the correct answer.
Explanation:
A reaction in which there occurs change in oxidation state of reacting species is known as an oxidation-reduction reaction.
(a)
Will be written as:

In this reaction, there occurs no change in oxidation state of reacting species. Hence, it is not an oxidation-reduction reaction.
(b) 
Will be written as:

Similarly here, there occurs no change in oxidation state of reacting species. Hence, it is not an oxidation-reduction reaction.
(c) 
Will be written as:

Here, also there occurs no change in oxidation state of reacting species. Hence, it is not an oxidation-reduction reaction.
(d) 
So, here there occurs change in oxidation state of Mg from 0 to +2 and oxidation state of H changes from +1 to 0. Hence, it is an oxidation-reduction reaction.
Thus, we can conclude that
is an oxidation-reduction reaction.
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Molar mass of A l 2 (S O 4 ) 3 = 342.15 g/mol
7.2g/ 342.15 g/mol
you can get number of moles.
then multiply that by12