pH= 7.44
What is the ratio of [H2PO4^-]/[HPO4^2-]
What does the pH become if 25% of the hydrogen phosphate ions are converted to the dihydrogen phosphate ion?
What does the pH become if 15% of the dihydrogen phosphate ions are converted to hydrogen phisphate ions?
Use the Henderson-Hasselbalch equation.
pH = pK2 + log [(base)/(acid)].
a) solve for the ratio directly.
b) choose a convenient number for the acid, use the ratio to calculate the base. Then decrease base by 25% and add that number of mols to acid. Recalculate pH using the HH equation.
c) similar to part b.
To answer these questions using the Henderson-Hasselbalch equation, we need to know the pK2 value for the H2PO4^-/HPO4^2- buffer system. Once we have that, we can calculate the ratio of H2PO4^- to HPO4^2- using the given pH value of 7.44.
Here's how you can calculate the ratio directly (part a):
1. Look up the pK2 value for the H2PO4^-/HPO4^2- buffer system. Let's say it is 7.21.
2. Plug the values into the Henderson-Hasselbalch equation:
pH = pK2 + log [(base)/(acid)]
7.44 = 7.21 + log [(base)/(acid)]
log [(base)/(acid)] = 7.44 - 7.21 = 0.23
3. Take the antilog of both sides to get the ratio:
[(base)/(acid)] = antilog(0.23)
To answer part b and part c, we need to calculate the pH after a certain percentage conversion of the buffer components. Let's take part b as an example:
1. Assume the initial ratio of [H2PO4^-] to [HPO4^2-] is given by the ratio calculated in part a.
2. Choose a convenient number for the concentration of acid (HPO4^2-). Let's say it is 1 mol.
3. Use the ratio to calculate the concentration of base (H2PO4^-). For example, if the ratio is 3:1, the concentration of base is 3 mol.
4. Decrease the concentration of base by 25% (since 25% of H2PO4^- is converted to HPO4^2-).
New concentration of base = 3 mol - (25% * 3 mol) = 3 mol - 0.75 mol = 2.25 mol.
5. Add the decreased base to the acid. You now have 2.25 mol of base and 1 mol of acid.
6. Calculate the new ratio: [(base)/(acid)] = 2.25 mol / 1 mol = 2.25.
7. Plug the new ratio and the pK2 value into the Henderson-Hasselbalch equation:
pH = pK2 + log [(base)/(acid)]
pH = 7.21 + log 2.25
Follow similar steps for part c, but this time decrease the concentration of acid by 15% and add it to the base.
Remember to use the same pK2 value throughout the calculations.