The answer to 42 is B and to 48 is C, please explain? For 48 I used E = h x freq

Question

The answer to 42 is B and to 48 is C, please explain? For 48 I used E = h x frequency then converted frequency to wavelength using c = frequency x wavelength then converted that to nanometers but I got a really small answer.

42. The standard reduction potential 2+, is-0.402 v (aq) A voltaic cell described by cd(s) 2 H (aq) 2+. H2(aq) (aq) has [Cd 0.900 M and a hydrogen pressure of 0.975 atm. Its cell potential at 25 oC is measured as E +0.192 V. What is the pH in the H IH2 halfcell? (A) 3.28 (B) 3.58 (C) 6.54 (D) 7.15

Explanation / Answer

Question 42.

Apply Nernst equation

Ecell = E º cell - 0.0592/n * log(Q)

Q = [Cd+2 ] P-H2 /([H+]^2)

n = 2 electrons, E º cell = 0 --0.402 = 0.402 V

Ecell = E º cell - 0.0592/n * log(Q)

0.192 = 0.402 - 0.0592/2 * log(Q)

0.192-0.40 = -0.0296*log(Q)

-0.21/(-0.0296) = log(Q)

logQ = 7.094

Q= 10^(7.094)

and we know that Q:

0.9 * 0.075 / ([H+]^2) = 10^7.094

[H+] = sqrt((0.9 * 0.075) / (10^7.094)) = 0.00007373129

pH = -log(0.00007373129) = 4

nearest answer is B

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