What Remains Conserved Out of Mass, Mole & Gram-atom During Any Process?5 min read

In this post, you will learn what remains conserved when (1) there is no chemical reaction, (2) there is a chemical reaction, and (3) there is a nuclear reaction.

Case 1: When there is no Chemical Reaction

Consider an example of the distillation column.

• First, apply the total mass balance

$\mathrm{100=D+B\;\;\;\;\;\;\;\;(1)}$

• Apply mass balance for benzene

$\mathrm{0.20\times100=0.50\times D+0.05\times B\;\;\;\;\;\;(2)}$

• From equations (1) & (2),

$\mathrm{D=33.33\;kg\;\;;\;\;\;\;\;\;\;\;B=66.67\;kg}$

Wt. of benzene (distillate side) $\mathrm{=33.33\times\frac{50}{100}=16.67\;kg}$

Wt. of benzene (bottom product side) $\mathrm{=66.67\times\frac{05}{100}=3.33\;kg}$

wt. of benzene (feed side) = wt. of benzene (distillate side) + wt. of benzene (bottom side)

$\mathrm{20\;kg=16.67+3.33=20\;kg \Rightarrow Mass\;is\;conserved}$

Molecular weight of benzene = 78 g/mol;  Molecular weight of toluene = 92 g/mol

Moles of benzene (feed side) $\mathrm{=\frac{20\;kg}{78\;g/mol}=256.41\;mol}$

Moles of benzene (distillate side) $\mathrm{=\frac{16.67\;kg}{78\;g/mol}=213.72\;mol}$

Moles of benzene (bottom product side) $\mathrm{=\frac{3.33\;kg}{78\;g/mol}=42.69\;mol}$

Moles of benzene (feed side) = Moles of benzene (distillate side) + Moles of benzene (bottom side)

$\mathrm{256.41=213.72+42.69=256.41 \Rightarrow Moles\;is\;conserved}$

$\mathrm{256.41=213.72+42.69=256.41}$

Conclusion

• When there is no chemical reaction, mass, moles & gram-atom remain conserved.

Critical Thinking

Here, benzene (C₆H₆) is not losing its identity. In the distillate and bottom side, benzene remains C₆H₆.

• Whenever a compound does not lose its identity then both mass, as well as mole balance, can be applied.

Case 2: When there is a Chemical Reaction

Consider an example of a simple reaction

• During chemical reactions, molecules lose their identity. However, the identity of elements remains intact.

Reactants side	Product side
1 gram-atom of C	1 gram-atom of C
4 gram-atom of H	4 gram-atom of H
4 gram-atom of O	4 gram-atom of O
Total 9 gram-atom	Total 9 gram-atom

$\mathrm{Gram‐atom\;(reactant\;side)\;=\;Gram‐atom\;(product\;side)}$

In a reaction, gram-atom remains conserved.

Total no. of moles (reactant side) $=1+2=3$; Total no. of moles (product side) $=1+2=3$

$\mathrm{Total\;moles\;(reactant\;side)\;=\;Total\;moles\;(product\;side)}$

• In this reaction, moles are conserved. Is it always true for all reactions? We’ll find out later.

Total mass of reactants $\mathrm{=16\;g+64\;g=80\;g}$; Total mass of products $\mathrm{=44\;g+36\;g=80\;g}$

$\mathrm{Total\;mass\;(reactant\;side)\;=\;Total\;mass\;(product\;side)}$

• In this reaction, gram-atom, mass & moles are conserved. Further, we will verify it by taking another reaction.

Consider another reaction

Reactants side	Product side
4 gram-atom of H	4 gram-atom of H
2 gram-atom of O	2 gram-atom of O
Total 6 gram-atom	Total 6 gram-atom

$\mathrm{Gram‐atom\;(reactant\;side)\;=\;Gram‐atom\;(product\;side)}$

Total mass of reactants $\mathrm{=4\;g+32\;g=36\;g}$; Total mass of products $\mathrm{=2\times18=36\;g}$

$\mathrm{mass\;of\;reactants\;=\;mass\;of\;products}$

Total no. of moles (reactant side) $\mathrm{=2+1=3}$; Total no. of moles (product side) $=2$

$\mathrm{moles\;of\;reactants\neq\;moles\;of\;products}$

Conclusions

• During a chemical reaction, mass & gram-atom are always conserved. However, no. of moles may or may not be conserved.

Critical Thinking

• Here, molecules are losing their identity during a chemical reaction. However, elements (such as C, H & O) are not losing their identity. So, mass & gram-atom remain to conserve.

Case 3: When there is a Nuclear Reaction (fusion & fission)

Consider a nuclear reaction

• Here, the identity of elements is no longer intact.

$\mathrm{Total\;moles\;(reactant\;side)\;\neq\;Total\;moles\;(product\;side)}$

• Weight of Deuterium (2 g/mol) + Weight of Tritium (3 g/mol) ≠ Weight of Helium (4 g/mol)

$\mathrm{Total\;mass\;(reactant\;side)\;\neq\;Total\;mass\;(product\;side)}$

$\mathrm{Gram‐atom\;(reactant\;side)\;\neq\;Gram‐atom\;(product\;side)}$

Conclusions

• During a nuclear reaction, total mass & energy before and after nuclear reaction remains conserved.

Critical Thinking

• When an element loses its identity then some mass can convert into energy or vice versa. So, total mass and energy remain conserved.
• This loss in mass can be related to energy using Einstein’s equation

$\mathrm{E=mc^2}$

• Where m is mass loss or defect mass.