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Question

Determine the molecular formula of an oxide of iron, in which the mass percent of iron & oxygen are 69.9 & 30.1 respectively.

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Solution

Molar mass of iron oxide:

Description

Given: 𝑚𝑎𝑠𝑠 % 𝑜𝑓 𝐹𝑒 𝑖𝑛 𝑖𝑟𝑜𝑛 𝑜𝑥𝑖𝑑𝑒=69.1%

𝑚𝑎𝑠𝑠% 𝑜𝑓 𝑜𝑥𝑦𝑔𝑒𝑛 𝑖𝑛 𝑖𝑟𝑜𝑛 𝑜𝑥𝑖𝑑𝑒=30.1%

Let the 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑖𝑟𝑜𝑛 𝑜𝑥𝑖𝑑𝑒=100 g

𝑚𝑎𝑠𝑠 % 𝑜𝑓 𝑒𝑙𝑒𝑚𝑒𝑛𝑡=massofelementmassofcompound×100

massofFe=massofcompound (ironoxide × mass %100

massofFe=100×69.1100=69.1 g

Similarly 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑜𝑥𝑦𝑔𝑒𝑛=30.1×100100=30.1 g

we know, 𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 𝑜𝑓 Fe=55.845 g mol1

𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑜𝑥𝑦𝑔𝑒𝑛=16 g mol1

& Number of 𝑚𝑜𝑙𝑒𝑠=MassofsubstanceMolarmassof substance

Number of 𝑚𝑜𝑙𝑒 𝑜𝑓 Fe=69.958.845=1.25

Number of 𝑚𝑜𝑙𝑒 𝑜𝑓 𝑜𝑥𝑦𝑔𝑒𝑛=30.116=1.88

Now,
Elements Fe O

Number of Moles 1.25 1.88 (for simplifying divide both by 1.25)

Ratio of moles 1 1.5 (It is not whole number so multiply both by 2)

Simple ratio 2 3

Hence, Molecular formula of Iron oxide Fe2O3

Final answer: molecular formula of Iron oxide is Fe2O3

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