Understanding Stoichiometry: Why We Cannot Compare Mass to Mass

In chemistry, stoichiometry is crucial for balanced chemical reactions. However, one common misconception arises from the inability to directly compare masses of reactants. This article delves into the reasons behind this limitation and explores why it is imperative to use moles instead of masses in chemical equations.

The Role of Atomic Weights

Atoms and even compounds have different atomic and molecular weights. For example, 2 grams of hydrogen (H2) contains approximately (6.023 times 10^{23}) molecules of H2, while 2 grams of oxygen (O2) contains about (frac{2}{32}) of (6.023 times 10^{23}) molecules of O2. This fundamental difference in atomic weights results in different numbers of molecules for each gram, leading to the infeasibility of directly comparing mass ratios.

The Importance of Using Moles

Chemical equations should be balanced in terms of moles, not mass. Balancing equations using moles ensures the conservation of atoms and molecules, adhering to the law of conservation of mass. For example, consider the reaction between hydrogen and oxygen to form water:

[frac{1}{2}H_2 frac{1}{2}O_2 rightarrow H_2O]

Here, 1 mole of H2 reacts with 0.5 moles of O2 to produce 1 mole of H2O. To simplify, the balanced equation is:

[2H_2 O_2 rightarrow 2H_2O]

This means that 2 moles of H2 react with 1 mole of O2 to form 2 moles of H2O.

An Example Using Ethane

To further illustrate, let’s consider the combustion of ethane (C2H6). The balanced chemical equation for this reaction is:

[C_2H_6 frac{7}{2}O_2 rightarrow 2CO_2 3H_2O]

This can be rewritten for easier understanding as:

[2C_2H_6 7O_2 rightarrow 4CO_2 6H_2O]

Here, 2 moles of ethane (C2H6) require 7 moles of diatomic oxygen (O2) for equivalence. Thus, 30 grams of ethane (the molar mass of ethane is 30 g/mol) requires 112 grams of diatomic oxygen (the molar mass of O2 is 32 g/mol, so 7 moles of O2 is 224 g, but we only need half of that for 2 moles of ethane, i.e., 112 g).

Conclusion

While it might seem simpler to compare masses directly, the fundamental differences in atomic and molecular weights necessitate the use of moles for accurate balancing of chemical equations. This ensures that the law of conservation of mass is upheld, leading to reliable and accurate chemical reactions.