Question

A mixture containing $ 100gm\text{ }H_{2}^{{}} $ ​ and $ 100gm\text{ }O_{2}^{{}} $ ​ is ignited so that water is formed according to the reaction, $ 2{{H}_{2}}+{{O}_{2}}\to 2{{H}_{2}}O. $ How much water will be formed?
(A) $ 112.5gm. $
(B) $ 50gm. $
(C) $ 25gm. $
(D) $ 200gm. $

Answer 1

Hint: We know that Stoichiometry is the study of the quantitative aspects of chemical reactions. Chemical equations are concise representations of chemical reactions. Mole is defined as the quantity of a substance that contains the same number of ultimate particles as are present in $ 12g(Carbon-12). $

Complete answer:
Stoichiometry deals with the numerical relationships of elements and compounds and the mathematical proportions of reactants and products in chemical transformations. Concentration is the amount of solute dissolved in a given amount of solution. There are different types of concentration units. Formula and molecular mass deal with individual atoms and molecules. Mole is the unit that relates the number of particles and mass.
In the question it is asked to calculate the molar mass of the water. The molecular formula of the water molecule is $ {{H}_{2}}O. $ Means in water molecules there are two hydrogens and one oxygen atom. Means one mole of water is going to be formed by the reaction of two moles of hydrogen and one mole of oxygen atom. The chemical reaction is given by;
 $ 2{{H}_{2}}+{{O}_{2}}\to 2{{H}_{2}}O. $
 $ 100gm({{H}_{2}})=\dfrac{100}{2}=50mole, $ similarly $ 100gm({{O}_{2}})=\dfrac{100}{32}=\dfrac{25}{8}mole. $ here divided by $ 2 $ and $ 32 $ is used since the atomic mass of the hydrogen and oxygen are $ 2 $ and $ 32 $ respectively.
According to stoichiometry the given reaction is $ 2{{H}_{2}}+{{O}_{2}}\to 2{{H}_{2}}O. $ since oxygen is limiting agent
 $ {{H}_{2}}Omole({{H}_{2}}O)=\dfrac{25}{4}. $
Therefore, weight of water becomes $ {{H}_{2}}O=\dfrac{25}{4}\times 8=112.5gm $

Additional Information:
Moles provide a bridge from molecular scale to real-world scale. One mole of molecules or formula units contain Avogadro number times the number of atoms or ions of each element in the compound. Each chemical equation provides information about the amount of reactants produced. Mole concepts enable us to solve stoichiometric problems involving mass relations of reactants and products in chemical reactions.

Note:
Remember that we can calculate the molar mass of any compound if we know the atomic weight of the individual atoms or elements present in the respective compound. First we have to calculate the molecular weight, later we can calculate the molar mass of the compound.