What is the molar mass of a compound with 0.5 atoms in its molecular formula? I'm trying to calculate the theoretical specific capacity of the $\ce{LiFePO4}$ cathode for a fully discharged state, let's say when the cathode has the following notation $\ce{Li_{0.5}FePO4}$. And I'm using the following formula:

$Q=\frac{nF}{3.6M}$ where F is the Faraday constant

The question is slightly related to this thread. The question is, what is the molar mass (M) of $\ce{Li_{0.5}FePO4}$.

Which one is the correct one for finding the specific capacity:


M = 157.751 g/mol, as if Lithium would be taken as a whole?
or
M = 154.285 g/mol, as if only half of Lithium would be considered?


Can anyone help me with this?
You should use 0.5 lithium atoms in the molecular mass. Alternatively you can multiply the formula indices by two to obtain $\ce{LiFe_2(PO_4)_2}$. This formula describes exactly the same compound but it's much more readable, for instance is now clear that the substance contains one $\ce{Li(I)}$, one $\ce{Fe(II)}$, one $\ce{Fe(III)}$ and two phosphate anions per $2n$ electrons transfered.

Here the value of $n$ is multiplied by 2 because you also have your stoichometry multiplied by 2. If you use $\ce{Li_{0.5}Fe(PO_4)}$ to calculate $M$ then the value of $n$ stays unchanged.