[DUDE778]

A biological cell is immersed in a 70. mM solution of NaCl at 37°C. The cell's membrane is permeable only to Na+ ions. When the system reaches equilibrium, a potential difference of 72.2 mV is measured between the inside and the outside of the cell, where the inside has the higher (more positive) potential. Calculate [Na+] inside the cell, assuming the amount of Na+ ions transferred is negligible compared to the total amount of Na+ ions in the solution.

Can someone give me a hand with this

[Padfoot]

a potential difference of 72.2 mV is measured

If you know the equilibruim potential, what piece of info does the Nernst eqn give you?

assuming the amount of Na+ ions transferred is negligible compared to the total amount of Na+ ions in the solution.

What can you say about the extracellular concentration?  Has it changed?

Also, don't forget your units are mM!

EDIT: spelling 

[DUDE778]

did i do it right..
72.2mV=61log[Na out] / [Na in]
i got 4.586M

[Padfoot]

did i do it right..

Yep
Though units are mM.

Also, don't forget your units are mM!

 Lordy Lord...   

72.2mV=61log[Na out] / [Na in]

Note that the Nernst eqn is: Eion= (-RT/zF)ln([ION]i/[ION]o)
which corresponds to -61log([ION]i/[ION]o) in this case.
(though mathematically your eqn is still right since log(a/b)=-log(b/a))

Edit: http://www.chemicalforums.com/index.php?topic=20830.msg78975#msg78975

That was 4 months ago 
Sheesh