What is the cell potential for the reaction mg(s+fe2+(aq? mg2+(aq+fe(s at 77 ? c when [fe2+]= 3.40 m and [mg2+]= 0.210 m . express your answer to three significant figures and include the appropriate units?

First, you need to calculate the standard cell potential using standard reduction potential from a textbook or online. Since Mg becomes Mg+2, magnesium is being oxidized because it is losing electrons, you need to flip its potential

Fe+2 + 2e- --> Fe                  potential= -0.44
Mg+2 + 2e- --> Mg                potential= -2.37

Cell potential= (-0.44) + (+2.37)= 1.93 V

Now, you need to use Nernst formula to get the answer. I have attached a PDF with the work.

They are all decomposition reactions.

You use the atomic number to find out how many protons and neutrons there are in the atom. you use the atomic mass to find out how many protons and electrons there are in the atom.