A fluoride electrode is used to determine fluoride in a 10-mL sample, The sample is diluted to 100mL with TISAB solution and measured with the electrode. The reading obtained is -226.5mV.

If a 2mL of 5.00 x 10^-2 M fluoride standard is added to the original sample and the solution is -238.6mV. If Nernstian response is assumed, what is the concentration of fluoride in the original sample?

I guess:

No. of mole of fluoride standard = (5.00 x 10^-2) x (2 x 10^-3) = 1 x 10^-4 mol

Since E = Q - 0.0592 log [F-],

0.2265 = Q - 0.0592 log[ ((1 x 10^-4) + (No. of mole of F- in sample) / (100 x 10^-3)]

0.2386 = Q - 0.0592 log[ (No. of mole of F- in sample)/ (100 x 10^-3)]

Is this flow correct? However, I do not how to solve these equations.