Emf of a Cell in an Open Circuit

Note that: The electromotive force (emf) is the potential difference of a source when no current is flowing. EMF has nothing to do with whether the terminals are connected to form a circuit or not. EMF is an inherent characteristic of a cell due to certain chemical reactions necessary for the driving of electrons to give rise to EMF. EMF is the voltage generated by a cell or by the magnetic force according to Faraday's Law.

So here

$$V=EMF$$ $$V' = EMF- Ir$$