What is the pH?

[BeatMCAT]

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What is the pH of a 10^-8 M HCl solution?

I thought it should be 8, but the answer key says 7. What am i missing here? Thanks in advance!

 

[indianjatt]

The concentration of hcl is too low so you have to account for the auto ionization of water. Also, if you just think about it, how can an acid be above a ph of 8 at 25 c?

 

[BerkReviewTeach]

What is the pH of a 10^-8 M HCl solution?

I thought it should be 8, but the answer key says 7. What am i missing here? Thanks in advance!

This question pops up in the Q and A section about every couple of years. It's a trick question that the general concensus says is not MCAT realistic, so don't worry about being tricked.

The key point to get is that water gives you 10^-7 M H+. When you add the 10^-8 M H+ you get from the HCl, the total molarity H+ is 1.1 x 10^-7, which would give you a pH of around 6.94. The pH of an acidic solution must be less than 7, but in this case it's barely less than 7.

Again, don't worry about this question. There's essentially a 0.0000000001% chance you'd see it on the MCAT. In your studies, it's important to recognize these unrealistic questions when you see them, because they can haunt you and undermine your progress. It's too easy to get stressed out by these when you should be shrugging them off.

 

[BeatMCAT]

Thanks a lot! I really appreciate it.