Alevel Chemistry Exam Question on Buffers..help for geniuses

Okay so this is an OCR Alevel Chem question ...I dont understand how and why the question is tackled. Help please??

H2PO4- (minus) is a weak acid and has the Ka value of 6.3 x 10^-8 mol dm-3. For the reaction it loses one proton. Write the equation for the reaction and express the Ka (1m).

Calculate the pH of a 0.010moldm-3 solution of NaH2PO4 . (2m)

A mixture of NaH2PO4 and Na2HPO4 acts as a buffer in fertilisers. Calculate the pH of a solution containing equal amounts of NaH2PO4 and Na2HPO4. (1m)

the answer is -log(ka (6.3x 10^-8))... why so though? why is pka the pH? can someone explain this probably really simple question please

Okay so this is the main toughie? lol its probably not .. help please?

Some students wish to make a buffer solution of pH 7 using NaH2PO4 and Na2HPO4. Suggest suitable masses of each solid to dissolve into 1.0dm^3 of water. (4m)

(edited 6 years ago)

Reply 1

M0nkey Thunder

13

You need to write the equation for the acid dissociating.

Rearrange the Ka equation to calculate the concentration of H+ ions to then calculate the pH from.

Where the concentration of weak acid and soluble salt of that acid are the same, you've reached the half-neutralisation point. The acid has dissociated halfway and so: Ka= [H+]. As a result pKa= pH.

For that last question, I believe you just need to figure out the ratio of the weak acid to its soluble salt. You already know the Ka value, can calculate the H+ concentration that you would have if the pH were to be 7. Just rearrange the Ka equation for that weak acid, find the ratio of [HA] and [A-] and you should be able to figure out suitable masses for both.

Reply 2

Diana :)

2

what paper is this question from?

Reply 3

GenuineHpLaptop

OP

9

Original post by M0nkey Thunder

You need to write the equation for the acid dissociating.

Rearrange the Ka equation to calculate the concentration of H+ ions to then calculate the pH from.

Where the concentration of weak acid and soluble salt of that acid are the same, you've reached the half-neutralisation point. The acid has dissociated halfway and so: Ka= [H+]. As a result pKa= pH.

For that last question, I believe you just need to figure out the ratio of the weak acid to its soluble salt. You already know the Ka value, can calculate the H+ concentration that you would have if the pH were to be 7. Just rearrange the Ka equation for that weak acid, find the ratio of [HA] and [A-] and you should be able to figure out suitable masses for both.

oo thankyou for clarifying the pka pH.

the answer for the 4 marker is put in the ratio 1.34 NaH2PO4 : 1 Na2HPO4 btw to anyone who works the answer? :smile:

)