Destroyer Gen Chem

[dentalduck15]

Q: What would be the approximate pH of a 1x10^-12M HCl solution?
Answer Choices: 12,2,7,5,-2


I need to understand the reasoning behind questions like this and how to approach them. Thanks.

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[shyfox9p]

Look at the exponent. If the coefficient is one, the pH is equal to the absolute value of the exponent. If the coefficient is not 1, apply the table. For example, if the conc. was 5*10^-11, the pH would be around 9.3. Memorizing the table for estimating logs will allow you to do these problems in seconds. Re-watch chad's video on logs and pH

 

[BrotherAli]

Look at the exponent. If the coefficient is one, the pH is equal to the absolute value of the exponent. If the coefficient is not 1, apply the table. For example, if the conc. was 5*10^-11, the pH would be around 9.3. Memorizing the table for estimating logs will allow you to do these problems in seconds. Re-watch chad's video on logs and pH

Wouldnt it be between 10-10.5?

How im doing it is:
1X10^-10=pH 10
3X10^-11=pH 10.5
1X10^-11=pH 11

So 5X10^-11 is in between 3X10^-11 and 10X10^-11 (1X10^-10)

 

[BobbyBeavis]

Wouldnt it be between 10-10.5?

How im doing it is:
1X10^-10=pH 10
3X10^-11=pH 10.5
1X10^-11=pH 11

So 5X10^-11 is in between 3X10^-11 and 10X10^-11 (1X10^-10)

You are correct, but @shyfox9p most likely just woke up and made a mistake considering his posting time

 

[goldenboy3.4]

The pH would be 7.

If you think about it, a pH of a 1 x 10 ^ -12 would be 12 => - log (1 x 10^-12) = 12. BUT, that tells us that we have a base, does HCl ever exist as a base? The concentration seems to be very dilute. Therefore, you would take into consideration of the molarity of the H3O+ from the water. H2O ---> H3O^+ + OH^-.

HCl dissolves completely, so the H+ ions would be 1 x 10^-12 as given, add it to the H3O+ (H+) ions from the water (1 x 10^-7). <<-- this came from Kw = Ka * Kb

So if you add 1 x 10^ -12 to 1 x 10^-7, it would be around 1 x 10^-7 still.

Take the - log of that value.

- log (1 x 10^-7) = 7.

 

[BobbyBeavis]

Awww snap I remember this problem now. Goldenboy is correct

 

[shyfox9p]

Wouldnt it be between 10-10.5?

How im doing it is:
1X10^-10=pH 10
3X10^-11=pH 10.5
1X10^-11=pH 11

So 5X10^-11 is in between 3X10^-11 and 10X10^-11 (1X10^-10)

Sorry for the confusion! It would be 10.3.

 

[Dotoday]

HCL is a monoprotic acid. So it releases only one proton. It will have the same concentration as HCl, because HCl is a strong acid. Then take the negative log.