Polyprotonic Acid question

[lDanny]

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I had a quick question about how to solve for pH and [H+} quickly.

1. Phosphoric acid (H3PO4) has Ka1 = 7.5 x 10-3, Ka2 = 6.2 x 10-8, and Ka3 = 4.2 x 10-13. What is the pH of 1.0 M phosphoric acid?

Ka2 and Ka3 are much smaller than Ka1 so it can be ignored. Is there a quick way of solving this without ICE table? TBR has the equation this equation pH=1/2pKa - 1/2log [HA] for weak acids.

2.H2SO4 has Ka1 = 1.0 x 103 and Ka2 = 1.2 x 10-2. What is the approximate hydrogen ion concentration of 1.0 M H2SO4?

H2SO4 is a strong acid so it completely dissociates. But what about the second acid? How do you quickly figure find the [H+]? thanks

 

[IntelInside]

I had a quick question about how to solve for pH and [H+} quickly.

1. Phosphoric acid (H3PO4) has Ka1 = 7.5 x 10-3, Ka2 = 6.2 x 10-8, and Ka3 = 4.2 x 10-13. What is the pH of 1.0 M phosphoric acid?

Ka2 and Ka3 are much smaller than Ka1 so it can be ignored. Is there a quick way of solving this without ICE table? TBR has the equation this equation pH=1/2pKa - 1/2log [HA] for weak acids.

2.H2SO4 has Ka1 = 1.0 x 103 and Ka2 = 1.2 x 10-2. What is the approximate hydrogen ion concentration of 1.0 M H2SO4?

H2SO4 is a strong acid so it completely dissociates. But what about the second acid? How do you quickly figure find the [H+]? thanks

For #1 I will use BR's shortcut:

pH = (pKa + pHif weak acid fully dissociated)/2
pH = (2.1 + 0)/2 = 1.05

 

[BerkReviewTeach]

Is there a quick way of solving this without ICE table? TBR has the equation this equation pH=1/2pKa - 1/2log [HA] for weak acids.

That formula is the shortcut version of the ICE box, and it works as long as Ka is small (which is the case here). That equation is manipulated one more step to get the easiest version of it.

As Intel point out, the BR short cut equation is:

pH = (pKa + pHif weak acid fully dissociated)/2

This works as long as pKa is at least two more than pHif weak acid fully dissociated).