4 replies to this topic

[Abhishek Thikpurlikar]

Sorry people, I am asking a very basic question, but when we take material balances, the mass balance always matches but this is not the case in case of mole balance.Why is this so?

[chenblue]

Hi,

We can write down balance-equations for any physical quantity, like mass, volume, mole of molecules, etc. But usually, we like using mass and volume, this is because they are measurable. And mass balance is most often used, because mass doesn't change with temperature and rxn.

[Andree]

And mass balance is most often used, because mass doesn't change with temperature and rxn.

and in the case of chemical reactions involved quantities expressed in moles are preserved only when stoichiometry is 1:1, hence it is more difficult to write a balance for moles than for mass

[Nayyer]

Sorry people, I am asking a very basic question, but when we take material balances, the mass balance always matches but this is not the case in case of mole balance.Why is this so?

hi,
this is nayyer.
when we talk about material balance, there are different quantities we can balance e.g. mass, moles, volume of a single compnent, a single atomic specie, or overall. now we know that we consider only that quantity about which we are pretty sure that it does not change in the process. if we talk about mass it does not change in any physical or chemical process, whether some reaction is occuring or some heat input output is involved. so we use mass balance most often. if we are sure about that no. of moles are not changing in the process, we can use mole balance. e.g. if some physical change is involed(moles dont change in physical process) or some chemical reaction in whcih moles of reactants and products are equal. and volume balance is not often used because volume changes if temperature of overall moles change in the process.

[Qalander (Chem)]

Dear Abhishek Thikpurlikar/Nayyer,

Although much is explained by our learned colleagues.I will like to put it in slightly different way

Whenever the processes under consideration have changes occurring

physically with volumetric changes and/or appreciable temperature changes

physically without volumetric changes and/or appreciable temperature changes

chemically associated with physical volumetric and/or appreciable temperature changes

chemically without appreciable volumetric changes and/or temperature changes

The mass balances are the easiest to re-conciliate/verify.

Mole balances require proper and comprehensive chemical reactions and balanced chemical reactions equations info.

The above signifies the reasons for differences observed as identified by you (Abhishek Thikpurlikar) in your opening post.

Hope this helps.
Regards
Qalander