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# How many moles of oxygen gas can be produced during the electrolytic decomposition of 180 g of water?A. 2.5 molesB. 5 molesC. 10 molesD. 7 moles

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.

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:
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.