Question

Phosphoric acid is a triprotic acid with the following pKa values: pka1= 2.148 pka2= 7.198 pka3=...

Phosphoric acid is a triprotic acid with the following pKa values: pka1= 2.148 pka2= 7.198 pka3= 12.375

You wish to prepare 1.000 L of a 0.0300 M phosphate buffer at pH 7.680. To do this, you choose to mix the two salt forms involved in the second ionization, NaH2PO4 and Na2HPO4, in a 1.000 L volumetric flask and add water to the mark. What mass of each salt will you add to the mixture?

What other combination of phosphoric acid and/or its salts could be mixed to prepare this buffer?

(Hint: Use the Henderson-Hasselbalch equation to get the molar ratio of Na2HPO4 to NaH2PO4 required, then the fraction of each form from the ratio. The total moles needed will be 1.000 L × 0.0300 M = 0.0300 moles. Use the formula mass to calculate the mass needed. (FM NaH2PO4 = 119.98; FM Na2HPO4 = 141.96))

Homework Answers

Answer #1

We know that,

pH for this buffer = pKa + log ([Na2HPO4] / [NaH2PO4])

Given,

pKa for H2PO4- = pKa2 = 7.198

and [Na2HPO4] + [NaH2PO4] = 0.03 M ------------- (1)

Applying the equation we get,

7.68 = 7.198 + log ([Na2HPO4] / [NaH2PO4])

=> log ([Na2HPO4] / [NaH2PO4]) = 0.482

=> ([Na2HPO4] / [NaH2PO4])) = 3.034

=> [Na2HPO4] = 3.034 x [NaH2PO4]

From (1)

3.034 x [NaH2PO4] + [NaH2PO4] = 0.03 M

=> [NaH2PO4] = 0.00744 M

and [Na2HPO4] = 0.0226 M

Volume of solution = 1 L

We know that,

Moles = Molarity x Volume (L)

=> Moles of NaH2PO4 = 0.00744 x 1 = 0.00744 moles

=> Mass of NaH2PO4 = 0.00744 x 119.98 = 0.893 g

Moles of Na2HPO4 = 0.0226 x 1 = 0.0226 moles

=> Mass of Na2HPO4 = 0.0226 x 141.96 = 3.21 g

Know the answer?
Your Answer:

Post as a guest

Your Name:

What's your source?

Earn Coins

Coins can be redeemed for fabulous gifts.

Not the answer you're looking for?
Ask your own homework help question
Similar Questions
Phosphoric acid is a triprotic acid with the following pKa values: pka1=2.148 pka2=7.198 pka3=12.375 You wish...
Phosphoric acid is a triprotic acid with the following pKa values: pka1=2.148 pka2=7.198 pka3=12.375 You wish to prepare 1.000 L of a 0.0500 M phosphate buffer at pH 7.780. To do this, you choose to mix the two salt forms involved in the second ionization, NaH2PO4 and Na2HPO4, in a 1.000 L volumetric flask and add water to the mark. What mass of each salt will you add to the mixture?
Phosphoric acid is a triprotic acid with the following pKa values: pKa1=2.148, pKa2=7.198, pKa3=12.375 You wish...
Phosphoric acid is a triprotic acid with the following pKa values: pKa1=2.148, pKa2=7.198, pKa3=12.375 You wish to prepare 1.000 L of a 0.0100 M phosphate buffer at pH 7.720. To do this, you choose to mix the two salt forms involved in the second ionization, NaH2PO4 and Na2HPO4, in a 1.000 L volumetric flask and add water to the mark. What mass of each salt will you add to the mixture? mass NaH2PO4? mass Na2HPO4? What other combination of phosphoric...
Phosphoric acid (H3PO4) is a triprotic acid with the following pKa values: pKa1 = 2.15 pKa2...
Phosphoric acid (H3PO4) is a triprotic acid with the following pKa values: pKa1 = 2.15 pKa2 = 7.20 pKa3 = 12.35 a). You wish to prepare a 1.000 L of a 0.0450 M phosphate buffer at pH 6.8. Which of the following salts do you use (PICK 2) and what mass of each salt will you add to the mixture? H3PO4 (M.W.= 97.995) KH2PO4 (M.W.=136.086) Na2HPO4 (M.W.=141.959) AlPO4 (M.W.=121.953)
Phosphoric acid (H3PO4) is a triprotic acid with the following pKa values: pKa1 = 2.15 pKa2...
Phosphoric acid (H3PO4) is a triprotic acid with the following pKa values: pKa1 = 2.15 pKa2 = 7.20 pKa3 = 12.35 a). You wish to prepare a 1.000 L of a 0.0450 M phosphate buffer at pH 6.8. Which of the following salts do you use (PICK 2) and what mass of each salt will you add to the mixture? H3PO4 (M.W.= 97.995) KH2PO4 (M.W.=136.086) Na2HPO4 (M.W.=141.959) AlPO4 (M.W.=121.953) b). A concentrated phosphate buffer known as “phosphate buffered saline” (or...
Phosphoric acid is a triprotic acid (Ka1 = 6.9× 10–3, Ka2 = 6.2× 10–8, and Ka3...
Phosphoric acid is a triprotic acid (Ka1 = 6.9× 10–3, Ka2 = 6.2× 10–8, and Ka3 = 4.8× 10–13). 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? A) pKa1= 2.16 B) pKa2=7.21 C) pKa3= 12.32 Calculate the pH of a buffer solution obtained by dissolving 24.0 g of KH2PO4(s) and 43.0 g of Na2HPO4(s) in water and then diluting to 1.00 L.
1. Phosphoric acid (H3PO4) is a polyprotic acid with pKa1 = 2.2; pKa2 = 6.8; pKa3...
1. Phosphoric acid (H3PO4) is a polyprotic acid with pKa1 = 2.2; pKa2 = 6.8; pKa3 = 12.4. What is the pH (to the nearest 0.1 pH unit) of a buffer made from mixing 90 mL of 0.1 M phosphoric acid and 10 mL of 0.1 M NaH2PO4? 2. The pKa of acetic acid is 4.76. What volume of 2 M HCl can be added to 100 mL of 0.1M acetate buffer at pH 4.76 before it is 90% titrated...
A weak acid H3A has pKa values of 1.95 (pKa1), 3.08 (pKa2), and 9.54 (pKa3). The...
A weak acid H3A has pKa values of 1.95 (pKa1), 3.08 (pKa2), and 9.54 (pKa3). The disodium salt of that weak acid was dissolved in deionized water to make 250.0 mL of a 0.025 M solution.  a) What was the pH of the resulting solution? b)What was the equilibrium concentration of H2A-?
You need to prepare a phosphoric acid buffer that has a buffer concentration of 0.1 M,...
You need to prepare a phosphoric acid buffer that has a buffer concentration of 0.1 M, a pH of 6.65, and taking in to account activity. First, ignoring ativiy, calculate [A-] and [HA] needed to prepare the buffer using the Henderson-Hasselbalch equation and the fact that [buffer]=0.1 M. (Hint: you have two equations and two unknowns). (pKa1=2.15, pKa2=7.09, pKa3=12.32) Please show your work and explain your steps.
Given that the stepwise dissociation constants for phosphoric acid are: Ka1 = 7.5×10-3; Ka2 = 6.2×10-8;...
Given that the stepwise dissociation constants for phosphoric acid are: Ka1 = 7.5×10-3; Ka2 = 6.2×10-8; Ka3 = 4.8×10-13 To prepare 1.20 L of a buffer solution having an ionic strength of 0.150 and a pH of 7.50 would require: Answer to 3 sig figs. a) (mass) of Na2HPO4(anhydrous) and b) (mass) of NaH2PO4(anhydrous).
1. Show the calculations required for preparing the buffers in this exercise. The pKa values for...
1. Show the calculations required for preparing the buffers in this exercise. The pKa values for H3PO4 are 2.15, 6.82, 12.32. Note that you’ll be using the pKa value of 6.82. Why? Method 1 a. Calculate the grams of NaH2PO4*H2O needed to prepare 100.0 mL of a 100mM solution. b. Calculate the amount of 1.00M NaOH you expect to add to adjust the pH of your NaH2PO4 solution to 7.0. Method 2 c. Using the Henderson-Hasselbalch equation, calculate the g...
ADVERTISEMENT
Need Online Homework Help?

Get Answers For Free
Most questions answered within 1 hours.

Ask a Question
ADVERTISEMENT