Question

A blood serum sample containing Na+ gave an emission signal of 4.27 mV. A small volume of concentrated standard Na+ was then added to increase the Na+ concentration by 0.102 M, without significantly diluting the sample. This "spiked" serum sample gave a signal of 8.09 mV. Find the original concentration of Na+ in the serum.

Answer is suppose to be 0.117 but I'm just not getting those numbers.
0.102M is responsible for 8.09-4.27 = 3.82 mv.
Then 0.102 x (4.27/3.82) = 0.114M

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