Nextstep Full Length 2: C/P #25

[akb5633]

QUESTION
What would be the approximate ratio of lactate to lactic acid in the final Lactated Ringer's solution after the initial preparation by student A?
(Relevant passage information: pH of solution = 6.6, Kb lactate = 2.5 x 10^-11)

a. 1:10
b. 1:1000
c. 100:1
d. 1000:1 <---correct answer


MY WORK
Ka x Kb = 1 x 10^-14 ---> Ka = (1 x 10^-14)/(2.5 x 10^-11) = 4 x 10^-4

pka = -logKa = -log (4 x 10^-4) ~ 3.4

...and then I blanked and continue to blank a day later. Anyone able to help me figure out where I go from here? The explanation at this point says:

"Solving for the ratio of [A-]/[HA] gives: lactate/lactic acid = 106.6/103.6 = 106.6 - 3.6 [my rounding of the pKa at 3.4 was a little off from there's] = 103 = 1000/1"


I'm just really uncertain of where the 106.6 and 103.6 are coming from. There is a table noting the concentration of lactate is 28 mM, but I'm not sure if that's relevant.

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[akb5633]

lol just figured it out myself, and holy ****...****ing nextstep needs to learn how to format their numbers.

Anyhow, for any other poor souls confused by their explanation, I just figured it out; you need to rearrange the Henderson-Hasselbalch equation to get the ratio of base to acid: [A-]/[HA] = 10^(pH-pka)
(those fools literally had it formatted as 106.6 instead of 10^6.6, and 103.6 instead of 10^3.6)

So FINAL ANSWER is [A-]/[HA] = 10^(6.6-3.6) = 10^3 = 1000:1 BOO YA

 

[theonlytycrane]

h.h. is in that form:

pH = pKa + log10([A-] / [HA])

 

[akb5633]

h.h. is in that form:

pH = pKa + log10([A-] / [HA])

I understand the H/H equation is pKa plus log base 10 of [A-]/[HA] and how it's rearranged. It was the actual formatting of the numerical aspects that threw me off; they literally had it typed as 106 and 103. Perhaps it should have come more intuitively to me that that meant ten to the sixth, and ten to the third power, but I feel as though they could have done a bit better with the formatting of those numbers.

 

[NextStepTutor_2]

Hi @akb5633

Can you PM me and let us know when/where you saw this problem? I checked the system and see

If you ever have an issue with the online resources, feel free to use our feedback button!

Good luck and keep studying!

 

[akb5633]

Hi @akb5633

Can you PM me and let us know when/where you saw this problem? I checked the system and see

View attachment 207757

If you ever have an issue with the online resources, feel free to use our feedback button!

Good luck and keep studying!

Hey, thanks for checking in. It may have been because I was taking the exam on a Mac computer, but anyway here's a screenshot of the explanation. For the most part, the formatting was okay, it was mainly just that last bit at the end.

 

[NextStepTutor_2]

Hey, thanks for checking in. It may have been because I was taking the exam on a Mac computer, but anyway here's a screenshot of the explanation. For the most part, the formatting was okay, it was mainly just that last bit at the end.

Thanks for the heads up. Looks like all the formatting is corrected now.

Good luck and keep studying!