$\ce{Pb(NO3)2 + 2KI -> PbI2 + 2KNO3}$

The answer says 26, but shouldn't it be 13? Because actually, the number of atoms involved in the reaction are 13, and once the reaction is completed, again, number of atoms is 13.
Please explain.

  • 3
    $\begingroup$ I believe the question is asking how many atoms are present in the written form of the equation, on both sides. In this case, you would disregard the actual reaction that is taking place, and simply count the number on the left and add it to the number on the right. $\endgroup$ Nov 15, 2017 at 17:40
  • 8
    $\begingroup$ To be honest, your answer is a lot better since the atoms on either side are the same, so 26 would be double counting... $\endgroup$
    – Zhe
    Nov 15, 2017 at 18:03
  • 3
    $\begingroup$ I totally agree with Zhe. One of the fundamental notions in chemistry is that chemical reactions don't create or destroy atoms. So you have as many atoms after a chemical reaction as before. // All in all, a badly worded question. $\endgroup$
    – MaxW
    Nov 15, 2017 at 21:01
  • $\begingroup$ Indeed an equation cannot represent the total number of atoms. There can be 13 n atoms. Which is not the 26 (n = 2) they intended. Very bad question. $\endgroup$
    – Alchimista
    Nov 18, 2017 at 13:29

2 Answers 2


The referenced atoms on both sides of the arrow are the exact same atoms? I like your thinking!

But, chemical equations do not refer to individual atoms.

The formula $\ce{Pb(NO3)2}$ can be read as "A molecule of this substance consists of six atoms of O, two atoms of N, ...". That is correct.

But the equation $\ce{Pb(NO3)2 + 2KI -> PbI2 + 2KNO3}$, strictly speaking, does not talk about single molecules. The numbers (including the 1's that are nor written) are called "stoichiometric coefficients". The equation means: "Any number of $\ce{Pb(NO3)2}$ molecules react with the double amount of $\ce{KI}$ molecules to yield ...".

Wikipedia supports my view:

In lay terms, the stoichiometric coefficient (or stoichiometric number in the IUPAC nomenclature) of any given component is the number of molecules that participate in the reaction as written.
In more technically precise terms, ...

... the stoichiometric coefficient is the number of molecules divided by the progress variable or extent of reaction.

The (dimensionless) "units" may be taken to be molecules or moles. Moles are most commonly used, but it is more suggestive to picture incremental chemical reactions in terms of molecules.

  • $\begingroup$ "But, chemical equations do not refer to individual atoms" unless the equation represents an elementary (act of) reaction, I guess. $\endgroup$
    – Wildcat
    Nov 15, 2017 at 21:05

they didn't specify counting either the reactants or products side alone. You were to count both the reactants & products side


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