# Problem: Phosphoric acid is a triprotic acid (Ka1 = 6.9 x 10-3, Ka2 = 6.2x 10-8, and Ka3 = 4.8 x10-13). To find the pH of a buffer composed of H2PO4- (aq) and HPO42- (aq), which pKa value would you use in the Henderson-Hasselbalch equation?Calculate the pH of a buffer solution obtained by dissolving 13.0 g of KH2PO4(s) and 26.0 g of Na2HPO4(s) in water and then diluting to 1.00 L.

###### FREE Expert Solution
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###### FREE Expert Solution

We’re being asked to calculate the pH of buffer composed of H2PO4-(aq) and HPO42-(aq). To calculate for pH, we’re going to use the Henderson-Hasselbalch equation:

We know that

So first have to figure out which Ka value we’re going to use for our calculation. Phosphoric acid (H3PO4) is a weak triprotic acid, meaning it can donate three protons (H+and it will have three equilibrium reactions.

Removing one H+ per step in H3PO4:

84% (216 ratings)
###### Problem Details

Phosphoric acid is a triprotic acid (Ka1 = 6.9 x 10-3, Ka2 = 6.2x 10-8, and Ka3 = 4.8 x10-13). To find the pH of a buffer composed of H2PO4- (aq) and HPO42- (aq), which pKa value would you use in the Henderson-Hasselbalch equation?

Calculate the pH of a buffer solution obtained by dissolving 13.0 g of KH2PO4(s) and 26.0 g of Na2HPO4(s) in water and then diluting to 1.00 L.

What scientific concept do you need to know in order to solve this problem?

Our tutors have indicated that to solve this problem you will need to apply the Polyprotic Acid concept. You can view video lessons to learn Polyprotic Acid. Or if you need more Polyprotic Acid practice, you can also practice Polyprotic Acid practice problems.

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Our tutors rated the difficulty ofPhosphoric acid is a triprotic acid (Ka1 = 6.9 x 10-3, Ka2 =...as high difficulty.

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Our expert Chemistry tutor, Dasha took 14 minutes and 16 seconds to solve this problem. You can follow their steps in the video explanation above.

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Based on our data, we think this problem is relevant for Professor Davis' class at UCF.
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