pH of weak acid's/bases

[DrRoyal Pains]

---

Members don't see this ad.

Weird question, probably a stupid one at that. I got the following question right on Chad's quiz, but I don't see how he got the right answer. The solution to the problem states H+ = 7 x 10 ^-4. He then says the pH is 5.2. Since 1 x 10^-4 has a pH of 4, shouldn't the pH of 7 x 10^-4 be below a pH of 4? I am pretty tired and probably overlooking something stupid in this question, but curious.

 

[RedRoze]

I think you're right, I got 3.15, because from what I know ph=-log[H+], 4=-log[1x10^-4], and 3=-log[7x10^-4]

 

[jwnichols21]

Which quiz is this from, and can you state the question? I'll check my notes for this.

 

[DrRoyal Pains]

What a doozy. This ish is going to bother me now 👎

Its on G5.08. The question is what is the pH of 0.1M HCN (Ka= 5.0 x 10 ^-10)

 

[Demps]

1x10^-4, pH = 4

10x10^-4, pH = 3

so 7x10^-4 between 3 and 4, but a little more closer to 3. So I think you are right.

 

[Demps]

Ok.

I am going to abbreviate some steps but I hope the numbers make sense to you.

5.0 x 10^-11 = 50 x 10^-12

square root of that is approximately ~7x10^-6

so pH = ~5.2 looks correct.

 

[DrRoyal Pains]

Ok.

I am going to abbreviate some steps but I hope the numbers make sense to you.

5.0 x 10^-11 = 50 x 10^-12

square root of that is approximately ~7x10^-6

so pH = ~5.2 looks correct.

What you have above is the way I did it. Looks like Chad has a little boo-boo on his website.

 

[Demps]

It's cool as long as you know your stuff 😀

 

[jwnichols21]

The reason is..

He took the square root incorrectly. It's the square root of 50 x 10 ^-12, which should be ~7 x 10 ^ -6 and not 7 x 10 ^-4. Thus, the answer is going to be lower than 6, or 5.2 ish.