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:

`" "E^@("V")`
`stackrel(color(white)(xxxxxxxxxxxxxxxxxx))(color(red)(larr))`

`Sn_((aq))^(2+)+2erightleftharpoonsSn_((s))" "-0.13`

`Cu_((aq))^(2+)+2erightleftharpoonsCu_((s))" "+0.34`
`stackrel(color(white)(xxxxxxxxxxxxxxxxxx))(color(blue)(rarr))`

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 `rArr`

`E_(cell)^@=+0.34-(-0.13)=+0.47"V"`

I have adopted the convention which is used in the UK.

I understand other conventions would reverse the sign of the `Sn^(2+)"/"Sn` 1/2 cell then add.

Other conventions I have seen write the 1/2 cells in the other direction and reverse the voltage.