Please show your work! You?ve just been hired as a research assistant and the st

Question

Please show your work!

You?ve just been hired as a research assistant and the student working in the lab before you did a terrible job of keeping a good lab notebook. There is a flask of phosphate buffer on the lab bench but no one knows what pH it is and all the pH electrodes are on back order from Fisher. Luckily, the previous student did write down the amount of H3PO4 (10.0g) they added to 2.5 L of water. Solve for the pH of this buffer solution so you know if you can use this for your experiments (*note you should always make your own reagent when working in Iab) Relevant acid/base reactions:

Explanation / Answer

Answer: Here we have given the 3 step reactions and three values of Ka and we know the final Keq would be equal to the multiplication of these 3 Ka values . concentration of H3PO4 = 10 /2.5 = 4M

final equation is like : H3PO4  -----> PO43- + 3H+ .......................................[A]

now we know that Pka1 = - log Ka1

now calculating ka1 we get = 7.586 * 10-3

and Ka2 = 7.585 * 10-8

and Ka3 = 2.1379 * 10-13

K eq = Ka1 * Ka2 * Ka3

now solving it we get

Keq = 1.23 * 10-13 =   [PO43-] [H+]3/[H3PO4]

now using the ICE method to calculate the concentration

1.23 * 10-13  = [x][x]3/4-x [here 4-x = 4]

now solvinf for x we get = 3.3 * 10-14

Hence the concentration of H+ ion is = 3.3 * 10-14 M

so Ph = - log[H+] = 13.48

Hence the PH would be = 13.48

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