Calculate the Molarity given kA and pH??? PLEASE HELP? You are instructed to cre

Question

Calculate the Molarity given kA and pH??? PLEASE HELP?

You are instructed to create 200. mL of a 0.30 M phosphate buffer with a pH of 6.5. You have phosphoric acid and the sodium salts NaH2PO4, Na2HPO4, and Na3PO4 available. (Enter all numerical answers to three significant figures.)

I know the acid component of the buffer is NaH2PO4 and the base component is Na2HPO4

THANK YOU!!!

• H3PO4(s) + H2O(l) H3O+(aq) + H2PO4?(aq)    

• Ka1 = 6.9???10?3

• H2PO4?(aq) + H2O(l) H3O+(aq) + HPO42?(aq)    

• Ka2 = 6.2???10?8

• HPO42?(aq) + H2O(l) H3O+(aq) + PO43?(aq)

• Ka3 = 4.8???10?13

Calculate the Molarity given kA and pH??? PLEASE HELP? You are instructed to create 200. mL of a 0.30 M phosphate buffer with a pH of 6.5. You have phosphoric acid and the sodium salts NaH2PO4, Na2HPO4, and Na3PO4 available. (Enter all numerical answers to three significant figures.) I know the acid component of the buffer is NaH2PO4 and the base component is Na2HPO4 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?

Explanation / Answer

buffer is NaH2PO4 and the base component is Na2HPO4

We have to consider Ka2,

pH = pKa2 + log(no.of moles of salt)/(no.of moles of acid)

6.25 = 8 - log6.2 + log(no.of moles of salt)/(no.of moles of acid)

log(no.of moles of salt)/(no.of moles of acid) = ?0.957

(no.of moles of salt) / (no.of moles of acid) = 0.11

The ratio of no.of moles of salt and acid are obtained.

Now, C = n/V

Csalt = (0.11 / 200) x1000 = 0.55 M

Cacid = (1/200) x1000 = 5.0 M

the molarity needed for the acid component of the buffer = 5.0 M

the molarity needed for the salt component of the buffer = 0.55 M

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