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Question: If 30ml of \[{{H}_{2}}\]and 20ml of \[{{O}_{2}}\] reacts to form \[{{H}_{2}}O\], what is left at the...

If 30ml of H2{{H}_{2}}and 20ml of O2{{O}_{2}} reacts to form H2O{{H}_{2}}O, what is left at the end of the reaction?
This question has multiple correct options:
A. 10ml of H2{{H}_{2}}
B. 5ml of H2{{H}_{2}}
C. 10ml of O2{{O}_{2}}
D. 5ml of O2{{O}_{2}}

Explanation

Solution

The reactant that fully reacts in the reaction is called reactant limiting or reagent limiting. The reactant that is not fully consumed in the reaction is called excess reactant.You should write the balanced reaction equation before solving the question. A balanced equation is an equation for a chemical reaction in which the number of atoms in the reaction for each element and the total charge for both the reactants and the products is equal.

Complete answer:
The Law of Definite Proportions, states that any chemical compound will always contain a fixed ratio of elements by mass. The Law of Definite Proportions is also sometimes called Proust's Law.
H2{{H}_{2}}and O2{{O}_{2}}react to form water.
The reaction used in question is: 2H2(g)+O2(g)→2H2O(g)2{{H}_{2}}(g)+{{O}_{2}}(g)\xrightarrow{{}}2{{H}_{2}}O(g)
2 moles of H2{{H}_{2}}combines with 1 mole of O2{{O}_{2}} to produce 2 moles of H2O{{H}_{2}}O.
In other words, we can say that 1ml of H2{{H}_{2}}reacts with 0.5ml of O2{{O}_{2}} to form 1ml of H2O{{H}_{2}}O.
Hence, when 30ml of H2{{H}_{2}}reacts with 15ml of O2{{O}_{2}}to form water it can be represented in a balanced chemical reaction as:
H2(g)+12O2(g)→H2O(g){{H}_{2}}(g)+\dfrac{1}{2}{{O}_{2}}(g)\xrightarrow{{}}{{H}_{2}}O(g)

Therefore, limiting reagent is H2{{H}_{2}}.
So, at the end of the reaction, 30ml of water and 5ml of O2{{O}_{2}}will be left.
So, the correct answer is “Option D”.

Note: The limiting reagent (also known as limiting reactant) in a chemical reaction is a reactant that is totally consumed when the chemical reaction is completed. The amount of product formed is limited by this reagent, since the reaction cannot continue without it.
Remember this reaction equation,
2H2+O2→2H2O2{{H}_{2}}+{{O}_{2}}\xrightarrow{{}}2{{H}_{2}}O
When 2 moles of H2{{H}_{2}}combines with 1 mole of O2{{O}_{2}} to produce 2 moles of H2O{{H}_{2}}O.