Question

How many moles of oxygen gas can be produced during the electrolytic decomposition of 180 g of water?
A. 2.5 moles
B. 5 moles
C. 10 moles
D. 7 moles

Answer 1

Hint: Water is made up of 2 volumes of hydrogen gas and 1 volume of oxygen gas. The reaction involved is: $\text{2}{{\text{H}}_{2}}+{{\text{O}}_{2}}\to 2{{\text{H}}_{2}}\text{O}$. Write the balanced reaction of formation of oxygen gas by electrolysis. This question is based on the mole concept. Then, find the moles of oxygen gas.

Complete answer:
Electrolytic decomposition is dissociation of a substance by the use of electricity. The reaction involved here is electrolytic decomposition is the reverse of the reaction of water as $\text{2}{{\text{H}}_{2}}\text{O}\to \text{2}{{\text{H}}_{2}}+{{\text{O}}_{2}}$. Let us solve this question step by step:
Step (1): Write the electrolysis reaction of oxygen gas and hydrogen gas-
At the cathode: $4{{\text{H}}^{+}}\left( \text{aq}\text{.} \right)+4{{\text{e}}^{-}}\to 2{{\text{H}}_{2}}\left( \text{g} \right)$
At the anode: $2{{\text{O}}^{2-}}\left( \text{aq}\text{.} \right)\to {{\text{O}}_{2}}\left( \text{g} \right)+4{{\text{e}}^{-}}$
Step (2): Molecular weight of oxygen gas is twice the weight of oxygen atom. One oxygen atom weighs 16 grams. So, the molar mass of ${{\text{O}}_{2}}$ is (16+16) or 32 grams.
Step (3): We will use this equation, $\text{2}{{\text{H}}_{2}}\text{O}\to \text{2}{{\text{H}}_{2}}+{{\text{O}}_{2}}$. The molar mass of water is 18 grams (one oxygen atom of 16 grams and two hydrogen atoms of 1 gram each).
Step (4): Using unitary method of calculation, $\text{2}{{\text{H}}_{2}}\text{O}\to \text{2}{{\text{H}}_{2}}+{{\text{O}}_{2}}$, 36 grams of water gives 1 mole of oxygen gas, then 180 grams of water gives $\dfrac{1\times 180}{36}$ or 5 moles of oxygen gas.
The correct answer of this question is 5 moles of oxygen gas can be produced during the electrolytic decomposition of 180 g of water.
So, the correct answer is “Option B”.

Note: Do not relate this question much deeper as a question of electrochemistry, to make this question difficult for you. This question can be easily solved by balancing and writing reactions. And use the mole concept. Don’t make mistakes while calculating using the unitary method.