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

100=D+B\;\;\;\;\;\;\;\;(1)

Apply mass balance for benzene

0.20\times100=0.50\times D+0.05\times B\;\;\;\;\;\;(2)

From equation (1) & (2),

D=33.33\;kg\;\;;\;\;\;\;\;\;\;\;B=66.67\;kg

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

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

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

20\;kg=16.67+3.33=20\;kg

• Mass is conserved.

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

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

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

Moles of benzene (bottom product side) =\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)

256.41=213.72+42.69=256.41

• Moles are conserved.

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

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

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

Total mass of reactants =16\;g+64\;g=80\;g ; Total mass of products =44\;g+36\;g=80\;g

• Mass is conserved.
• 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

Gram atom (reactant side) = Gram-atom (product side)

Total mass of reactants =4\;g+32\;g=36\;g ; Total mass of products =2\times18=36\;g

mass of reactants = mass of products

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

Total moles of reactant \neq Total moles of product

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 more intact.

Total moles (reactant side) ≠ Total moles (product side)

Gram-atom (reactant side) ≠ Gram-atom (product side)

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

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

E=mc^2

• Where m is mass loss or defect mass.