# Chemistry

You are instructed to create 500. mL of a 0.25 M phosphate buffer with a pH of 7.7. You have phosphoric acid and the sodium salts NaH2PO4, Na2HPO4, and Na3PO4 available. (Enter all numerical answers to three significant figures.)

H3PO4(s) + H2O(l) equilibrium reaction arrow H3O+(aq) + H2PO4−(aq)
Ka1 = 6.9 ✕ 10−3
H2PO4−(aq) + H2O(l) equilibrium reaction arrow H3O+(aq) + HPO42−(aq)
Ka2 = 6.2 ✕ 10−8
HPO42−(aq) + H2O(l) equilibrium reaction arrow H3O+(aq) + PO43−(aq)
Ka3 = 4.8 ✕ 10−13

Questions:
What is the molarity needed for the acid component of the buffer?

What is the molarity needed for the base component of the buffer?

How many moles of acid are needed for the buffer?

How many moles of base are needed for the buffer?

How many grams of acid are needed for the buffer?

How many grams of base are needed for the buffer?

How do I do this and what are the answers?

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asked by Sarah
1. See your problem below. It's quite similar. Use HH equation to solve for (acid)/(base)
pH = pK2 + log (base/acid) ......(base/acid) = b/a
Plug in the values and solve for b/a. That's equation 1.
Equation 2 is a + b = 0.25M
Solve for (a) and (b). That gives you the molarity needed for the acid and the base.

Using M = mols/L, convert moles needed for acid or base convert to moles need for acid and base. You have M and you have L, solve for mols needed for acid and base. Remember acid is mols H2PO4^2- and base is HPO4^-.

After you have moles needed for base and acid, then
grams need = moles x molar mass of the salt.

Post your work if you get stuck.

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posted by DrBob222

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