What is the standard emf of a galvanic cell made of a Sn electrode in a 1.0 M #Sn(NO_3)_2# solution and a Cu electrode in a 1.0 M #Cu(NO_3)_2# solution at 25°C?
You need to look up standard electrode potentials and list them -ve to +ve:
You can see that the more +ve 1/2 cell will take in the electrons so the 1/2 cell reactions proceed in the direction indicated by the arrows.
Emf is an experimentally measured quantity and must always have a +ve value.
If you try to measure the emf of a cell and get a -ve reading, it means you have connected the voltmeter to the wrong terminals.
So to get the emf of the cell, always subtract the least positive potential from the most positive potential
I have adopted the convention which is used in the UK.
I understand other conventions would reverse the sign of the
Other conventions I have seen write the 1/2 cells in the other direction and reverse the voltage.