## HW 14.107

$E_{cell} = E_{cell}^{\circ}-\frac{RT}{nF}\ln Q$

Lizzie Zhang 2C
Posts: 31
Joined: Fri Sep 29, 2017 7:05 am

### HW 14.107

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
Posts: 25
Joined: Thu Jul 13, 2017 3:00 am

### Re: HW 14.107

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.

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