21.5 Exercise 4

What is the $\text{emf}_{\text{x}}$ of a cell being measured in a potentiometer, if the standard cell’s emf is 12.0 V and the potentiometer balances for $R_{\text{x}} = 5 . \text{000} \Omega$ and $R_{\text{s}} = 2 . \text{500} \Omega$?