HW 14.107

[Lizzie Zhang 2C]

For the question 107: What range (in volts) does a voltmeter need to have to measure pH in the range of 1 to 14 at 25 C if the voltage is zero when pH 7?

In the solution it used the equation of E°=RT/nF ln(K)
However, for K it used the ratio: [H]/[OH], and I wonder why K is expressed like that.
And why can't we use the equation E=E°+(0.0592v) X pH?

Thanks!

[zanekoch1A]

They use [H]/[OH] because this is relating the concentrations of products and reactants but only considering the compounds (H30+ and OH-) that matter for PH. It is a way of simplifying the problem's calculations I think. You cannot use the equation you listed E=E°+(0.0592v) X pH? because the question is asking about the volts at equilibrium and E=0 at equilibrium.