Calculating pH

[MedPR]

So for a strong acid I thought you could set the concentration of the acid equal to the concentration of H+ and use pH=-log[H+]?

What is the pH of a 5.0*10^-8 aqueous solution of H2SO4 at room temperature?

A. 8.3
B. 7.3
C. 6.8
D. 6.0

Answer in white: C

I did pH=-log(5*10^-8) and came out with 7.3, but that's not the right answer.

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[milski]

Only if the concentration of the strong acid is high enough. If you have only 5E-8 of [H] coming from H2SO4 water will be the major contributor of [H] and the pH will be 7 or just a tiny bit below 7, since the strong acid is contributing something too. From the choices below I'd go with C, 6.8

 

[MedPR]

Only if the concentration of the strong acid is high enough. If you have only 5E-8 of [H] coming from H2SO4 water will be the major contributor of [H] and the pH will be 7 or just a tiny bit below 7, since the strong acid is contributing something too. From the choices below I'd go with C, 6.8

Oh right. 7.3 would mean that the acid is raising the pH of the water, which doesn't make any sense.

Thanks.

 

[bajoneswadup]

Oh right. 7.3 would mean that the acid is raising the pH of the water, which doesn't make any sense.

Thanks.

So basically for every acid/base question I can't just
plug and chug? ughhhh.

 

[BRIAN1986]

Agree very right

 

[MrNeuro]

Only if the concentration of the strong acid is high enough. If you have only 5E-8 of [H] coming from H2SO4 water will be the major contributor of [H] and the pH will be 7 or just a tiny bit below 7, since the strong acid is contributing something too. From the choices below I'd go with C, 6.8

what does the concentration have to be greater than?

1.0 x 10^-7?

 

[ljc]

what does the concentration have to be greater than?

1.0 x 10^-7?

Yes.

Water is 1 x 10^-7, so adding acid to an aqeous solution will never get you a pH above 7.0

 

[milski]

Probably around 1e-6. It really depends on how precise you want to be about your calculation. At 10 times higher than water's contribution you'll start seeing changes and will be better off to treat it as an equilibrium problem for water. For most multiple choice questions the answer will be fairly obvious - something a bit below 7.

 

[DocGodfather]

Probably around 1e-6. It really depends on how precise you want to be about your calculation. At 10 times higher than water's contribution you'll start seeing changes and will be better off to treat it as an equilibrium problem for water. For most multiple choice questions the answer will be fairly obvious - something a bit below 7.

Nice explanations, milski. I get a little fuzzy at times with calculations like this. Need to start brushing up before I take the MCAT. Thanks. 👍

 

[deleted357625]

So you have pH of water that is 1*10^-7
You have a concentration of Acid - very small amount 5*10^-8. That is a minute amount of acid, which will make the water a little below 7.

If you just calculated the pH of 5.0*10^-8 that would be above 7 and that wouldn't make since since water's pH is 7 and adding an acid will not raise the pH.

 

[dmf2682]

And this is why I hang out here. Learned something new. Thanks all.

 

[gettheleadout]

So you have pH of water that is 1*10^-7
You have a concentration of Acid - very small amount 5*10^-8. That is a minute amount of acid, which will make the water a little below 7.

If you just calculated the pH of 5.0*10^-8 that would be above 7 and that wouldn't make since since water's pH is 7 and adding an acid will not raise the pH.

Torshi's bold point here is probably the most important to remember for these types of problems, IMO. If the pH doesn't make sense, you're ranking the present equilibria incorrectly (i.e. it isn't just H2SO4 -> H+ + HSO4- controlling the [H+])

 

[typicalindian]

Torshi's bold point here is probably the most important to remember for these types of problems, IMO. If the pH doesn't make sense, you're ranking the present equilibria incorrectly (i.e. it isn't just H2SO4 -> H+ + HSO4- controlling the [H+])

Yes very very true. If there's one thing I've learned from doing all these passages and stuff it's been to use common sense before plugging and chugging. Although its easier said than done when your under timed conditions haha

 

[deleted357625]

Yes very very true. If there's one thing I've learned from doing all these passages and stuff it's been to use common sense before plugging and chugging. Although its easier said than done when your under timed conditions haha

Yes, they try to make you think it's a no-brainer, avoid the idea of plug and chug because most of the time they are tricking you.