Ek ice-l6 q#138

[qtsjoe]

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"NH3 has a Kb of 1.8x10^-5. Which of the following has a Ka of 5.6x10^-10"
A)NH3
B)NH4+
C)NH2-
D)H+

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I understand given a Kb you can find Ka by dividing Kw/Kb...I just don't understand how I can make the connection between these choices and a value (Kw/Kb) or the given value Ka.

Thanks

 

[MedPR]

"NH3 has a Kb of 1.8x10^-5. Which of the following has a Ka of 5.6x10^-10"
A)NH3
B)NH4+
C)NH2-
D)H+

---

I understand given a Kb you can find Ka by dividing Kw/Kb...I just don't understand how I can make the connection between these choices and a value (Kw/Kb) or the given value Ka.

Thanks

So Kw=Ka/Kb for conjugates. The conjugate acid of NH3 is NH4+.

10^-14=1.8*10^-5 * Ka of NH4.

Round 1.8 to 2, then divide. (1*10^-14)/(2*10^-5) = (1/2)*(10^-9) = 0.5*10^-9 = 5*10^-10.

Answer is B.

You might also remember from your undergrad chemistry that the pKa of NH4Cl is about 9.

-log(5.6*10^-10) = -log(-10) - (log2+log3) = 10 - (.3+.48) = 9.22.

 

[qtsjoe]

Thanks,

If I understand correctly then... When we make the relation between Ka, Kb, Kw (Kw=KaKb) we are referring to the context of the conjugates? In perspective then wouldn't NH4 be the only suitable answer for the question by definition of making the relation to Kw=KaKb?

Can we find the Ka of A or C (D is irrelevant?)

 

[MT Headed]

Thanks,

If I understand correctly then... When we make the relation between Ka, Kb, Kw (Kw=KaKb) we are referring to the context of the conjugates?

👍

Can we find the Ka of A or C (D is irrelevant?)

No, although you can deduce that C < A < B because if you pull one proton off, then the molecule is less likely to give up another proton. This kind of relationship is sometimes tested.

 

[qtsjoe]

got it, thank you

 

[MedPR]

Thanks,

If I understand correctly then... When we make the relation between Ka, Kb, Kw (Kw=KaKb) we are referring to the context of the conjugates? In perspective then wouldn't NH4 be the only suitable answer for the question by definition of making the relation to Kw=KaKb?

Can we find the Ka of A or C (D is irrelevant?)

In this case NH4 is the right answer, but you don't always have to be able to calculate something in order to pick it as the right answer. Say they told you the pKb of NH3 is 4.5 and asked you, which of the following has a Ka of 10^6?

A. NH4
B. NaOH
C. NH2-
D. HCl

You can't calculate the Ka for NaOH or for HCl, but you know that whatever the answer is, it's significantly more acidic than NH4. So the answer would be D.