Hi there! So I am studying for my exam tomorrow and I understand how to solve th

Question

Hi there!
So I am studying for my exam tomorrow and I understand how to solve this problem once it's set up, but what I'm confused about it how to get the 1.0x10^-14 to divide with the 1.8x10^-5 given in the problem. We get a formula sheet on the test but the 1.0x10^-14 isn't provided anywhere so I was just curious. Can you help me? Thank you! MULTIPLE CHoICE. Choose the one alternative that best completes the statement or answers the question. 1) calculate the pH of a solution formed by mixing 2500 ml of 0.15 MNH4cl with 100.0 mL ofo.20 M NHb. The Kb for NH3 is 1.8 x 10 E) 9.25 D) 9.13 C) 4.74 A) 9.53 B) 898

Explanation / Answer

Note that this is a buffer

- weak base, NH3 is present

- conjugate acid, NH4+ is present

so

No need to use Kw, values

The buffer equation ( basic buffer)

pOH = pKb +log(NH4+/NH3)

now

recall that

pKb = -log(Kb) = -log(1.8*10^-5) = 4.75

so

pOH = 4.75 + log(NH4+/NH3)

we need to recalculate amounts, since V changes

[NH4+] = M1*V1/(V1+V2) = 250*0.15/(250+100) = 0.10714

[NH3] = M2*V2/(V1+V2) = 100*0.2/(250+100) = 0.05714

substitute

pOH = 4.75 + log(0.10714/0.05714 )

pOH = 5.02301

pH = 14-pOH = 14-5.02301 = 8.978

best anaswer is 8.98 (B)

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