# Finding salt concentrations :

1. Sep 23, 2004

### Gata

can someone please help me with this problem, its giving me lots of trouble to solve :

(a) Suppose you wanted to make a buffer of exactly ph 7.00 using KH2PO4 and Na2HPO4. If you had a solution of 0.1M KH2PO4, what concentration of Na2HPO4 would you need?

(b)Now assume you wish to make a buffer at the same ph, using the same substances, but want the total phosphate molarity (HPO42-) + (H2PO4-) to equal 0.3 What concentrations of KH2PO4 and Na2HPO4 would you use?

2. Sep 23, 2004

### Gokul43201

Staff Emeritus
What have you tried so far ? What ideas do you have ?

Tell us what you've tried and we'll guide you the rest of the way.

3. Sep 23, 2004

### Gata

i tried the henderson/hasselbalch equation :
the salt : Na2hpo4
the acid :kh2po4
ph= 7.0
pka ? not sure
not sure how to proceed on the second part or how to finish the first part

4. Sep 24, 2004

### chem_tr

If you want to find pH, then you should use pKa of the acid in Henderson-Hasselbach equation.
In your second question, you don't know the molar ratio of conjugate base to acid. The rest is known, just find it.

5. Sep 29, 2004

### Gata

question

its been said that the pka of strong bases are so close to 1 they ( the pka ) can be ignored.

Is this true ?

6. Sep 29, 2004

### chem_tr

Yes that's correct, but it is invalid for this question, as phosphate species other than ortho-phosphoric acid (H3PO4) need pKa values. If you were not given that data, it is nearly not possible to solve the H-H equation. Please find the pKa values for them, and solve this question. It is not that difficult. But when you think you're lost, keep asking.

7. Sep 29, 2004

### Gata

thanks

i have already solved the problem i was just asking a general question
thanks chem_tr

8. Oct 1, 2004

### Gata

new question

I was given this lab problem calc the ph values and draw the titration curve for the titration of 500ml of 0.010m acetic acid (pka 4.76) with .010m KOH.
I have to give two points the starting ph and the equivalence point.

I found the equivalence point to be the pka ( where ph=pka ).
I know i have to calc the second point using the KA Im just not sure how to plot the x-axis and y-axis

for the y-axis i know i have to plot ph
but for the x-axis is it KOH (eq ) or what ?
im not sure
can someone help me with this one ?

9. Oct 3, 2004

### chem_tr

Well, the starting pH will be the pH of the acetic acid solution with no KOH added, I am sure you are not asking this. What you're asking instead is simple, too; just plot the curve between titer (mL of KOH solution) and pH, on x and y axis, respectively.