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# pH of 35.2 wt % HCl?2

## pH of 35.2 wt % HCl?

(OP)
Hi guys. Mech engr here, in over my head again, as usual. How do I estimate the pH of a 35.2 weight % HCl solution? Is this acid too strong to use the old pH = -log[H+] and I have to take the log of the activity (which I have no idea how to do)?

Thanks guys! Pete

### RE: pH of 35.2 wt % HCl?

pH = -1.1

HCL

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

### RE: pH of 35.2 wt % HCl?

(OP)
Awesome. Thanks brother! Pete

### RE: pH of 35.2 wt % HCl?

And, yes, it is too strong for the simple pH equation. pH is defined as 0 to 14. Notice it is -1.1.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

### RE: pH of 35.2 wt % HCl?

(OP)
Right. That's why I was asking about the activity. Just from a quick Interpipes search, apparently for strong acids, a guy has to use pH=-log[a+] where a = activity of the hydrogen ion, I guess because strong acids do't completely dissociate. (ME in over his head again.) I have no idea how to find activity. It's not in my freshman chem textbook.

Source: http://antoine.frostburg.edu/chem/senese/101/acidb...

### RE: pH of 35.2 wt % HCl?

Most manufacturers would list the pH as being 0 to 1 or <1. Information may be obtained from the material safety sheets.

http://www.cleartech.ca/ckfinder/userfiles/files/H...

Negative pH values may be possible, but would be unusual in practice. There are some complications in high molarity acid solutions that make pH calculations from acid molarity inaccurate and difficult to verify experimentally:

Even strong acids don't dissociate completely at high concentrations. Some of the hydrogen remains bound to the chlorine, making the pH higher than you'd expect from the acid molarity.

Because there are so few waters per acid formula unit, the influence of hydrogen ions in the solution is enhanced. We say that the effective concentration of hydrogen ions (or the activity) is much higher than the actual concentration. The usual general chemistry text definition of pH as -log [H+] (negative the logarithm of the hydrogen ion molarity) is better written as pH = - log aH+ (negative the logarithm of the hydrogen ion activity). This effect is very strong, and makes the pH much lower than you'd expect from the acid molarity.

If you were to dip a glass pH electrode into the 12 M HCl solution to actually measure the pH, you would get a pH that was higher than the true pH. This well-known defect in glass pH electrode measurements is called the "acid error"; it is sensitive to experimental conditions and difficult to correct for.

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