Now a trivial answer would be that the current is 0.
V= E-Ir (where v is terminal voltage, e is emf, I is current, and r is internal resistance) Since i=0, V=E
But one thing I believe we are neglecting here is that ohms law does not work when either of three quantities is 0 (in this case the current). Simply because I can write V=IR (where R is external resistance), this would mean v=0 which is not the case.
Also there's a way I've figured out to solve this but in that case we assume the external resistance to be infinity in open circuit
V=IR --> I=V/R (where R is very close to infinity)
Since R is very much greater than r we can ignore the value of r, this would imply
E= (V/R)(R) --> E=V
Is this a correct way to tackle this problem or do I have a flaw in my understanding?