[Jenran]

I'm currently studying chemistry from textbook Chemistry (from OpenStax) and I'm stuck on this, trying to make sense out of it:





Molecular formula is obtained by molar mass of element multiplied by quantity of that element and at end, adding together all elements like this:





Empirical mass formula from what I read in this part of the book, is obtained basically in the same way, meaning that molecular mass for CH₂O is 30 amu and empirical mass formula is also 30 amu, but in this book it is written that molecular mass is 180, I don't know what's going on, from where such a number? How is it even possible?

[Borek]

You are misreading the book and/or misunderstanding/mistaking molecular formula with empirical formula.

Empirical formula tells you what is ratio of numbers of atoms in a molecule, it doesn't say anything about molecular mass and molecular formula (which can be any integer multiply of the experimental formula).

[Jenran]

Empirical formula tells you what is ratio of numbers of atoms in a molecule, it doesn't say anything about molecular mass and molecular formula (which can be any integer multiply of the experimental formula).

I don't know if I am correct, but I think I realized that empirical formula mass is made up from empirical formula which is only a ratio of numbers of atoms, which means that "real" molecule is not CH₂O but in this case, if molecular mass equals 180, it's actually C₆H₁₂O₆.

But is CH₂O and C₆H₁₂O₆ same molecule? It's confusing.

[Borek]

I don't know if I am correct, but I think I realized that empirical formula mass is made up from empirical formula which is only a ratio of numbers of atoms, which means that "real" molecule is not CH₂O but in this case, if molecular mass equals 180, it's actually C₆H₁₂O₆.

Sounds OK to me.

But is CH₂O and C₆H₁₂O₆ same molecule? It's confusing.

No, it is not the same molecule. But CH₂O is the empirical formula of C₆H₁₂O₆.

To make things less confusing consider that: CH₂O is an empirical formula, but real formula of a real compound is (CH₂O)_n. It may happen that n=1, in which case CH₂O is a real formula of a compound.

This is not invented to make life harder, this is just reflecting the reality Take two real and quite common compounds, one with a real, molecular formula CH₂O (formaldehyde AKA methanal) and the second with a real, molecular formula C₂H₄O₂ (acetic acid). If you make the elemental analysis you will find what are ratios of C:H:O, and you will find out both compounds have the same empirical formula of (CH₂O)_n. If you determine molar masses separately you will find out it is 30 g/mol for the first compound and 60 g/mol for the latter, so in the first case n=1 and in the second n=2. You can't determine n without knowing the molar mass.

[Jenran]

I understand, thank you!