Question

When steam (hot water) is passed over iron, hydrogen gas and iron (III) oxide is formed. What mass of steam would be needed to react completely with 100g of iron?

Answer 1

The balanced chemical reaction can be used to determine the molar relationship between substances. To solve the given problem, first find the balanced equation which correctly describes the reaction and then identify the given information and what we need to find. Then by using proper conversion factors, the value of an unknown quantity can be calculated. The calculations involving coefficients of molecules are known as mole-mole stoichiometry.

Complete answer:
As per question, iron reacts with steam to form iron oxide along with the liberation of hydrogen gas. The given process can be represented with the help of a chemical equation as follows:
$Fe+{{H}_{2}}O(g)\to F{{e}_{2}}{{O}_{3}}+{{H}_{2}}(g)$
We first need to balance the given chemical reaction i.e., the number of atoms of each element in the product and reactant must be balanced.
Step-1: Check the number of atoms of each element in the given reaction.

Element	Reactant	Product	Balanced
$Fe$	1	2	No
$H$	2	2	Yes
$O$	1	3	No

Step-2: In order to balance the given chemical reaction, we need to add required coefficients to each molecule to balance the number of atoms of each element. The balanced chemical reaction will be as follows:
$2Fe+3{{H}_{2}}O(g)\to F{{e}_{2}}{{O}_{3}}+3{{H}_{2}}(g)$
Now, we can compare the moles of the molecules using the mole-mole stoichiometric ratio to calculate the mass of steam required to react with the given mass of iron.
Given mass of iron $=100g$
Molar mass of iron $=55.8gmo{{l}^{-1}}$
Number of moles of iron $n=\dfrac{\text{given mass}}{\text{molar mass}}$
$\Rightarrow n=\dfrac{100}{55.8}$
$\Rightarrow n=1.8\text{ moles}$
Since, 2 moles of iron react with $\Rightarrow$ 3 moles of steam (water)
Therefore, 1.8 moles of iron will react with $\Rightarrow \dfrac{3}{2}\times 1.8=2.7$ moles of steam.
We know that number of moles of a molecule is $x=\dfrac{\text{given mass}}{\text{molar mass}}$ and the molar mass of water is $18gmo{{l}^{-1}}$ . So, the mass of steam corresponding to 2.7 moles will be as follows:
${{m}_{steam}}=18\times 2.7$
$\Rightarrow {{m}_{steam}}=48.6g$
Hence, we can conclude that the mass of steam needed to react completely with 100g of iron is 48.6g.

Note:
It is important to note that subscripts tell us the number of atoms of each element in a molecule whereas coefficients tell us the number of molecules in the reaction. While balancing the chemical reaction, we are not allowed to change the subscripts of the molecule as it will change the chemical composition of the molecule. We are only allowed to add coefficients to the molecule in order to balance a chemical reaction.