A solution containing 45.00 mL of 0.0500 M metal ion buffered to pH = 11.00 was

Question

A solution containing 45.00 mL of 0.0500 M metal ion buffered to pH = 11.00 was titrated with 0.0400 M EDTA. Answer the following questions and enter your results with numerical value only. Calculate the equivalence volume, Ve, in milliliters. 56.2 You are correct. Your receipt no. is 157-1182 Help: Receipt Previous Tries Calculate the concentration (M) of free metal ion at V = 1/2 Ve. 0.0154 You are correct. Your receipt no. is 157-4788 Help: Receipt Previous Tries Calculate the fraction (Y4-) of free EDTA in the form Y4. Keep 2 significant figures. 0.81 You are correct. Your receipt no. is 157-603 Help: Receipt Previous Tries If the formation constant (Kf) is 1012.00. Calculate the value of the conditional formation constant Kf’ (=Y4- * Kf) and enter your result as scientific notation form. 8.1×1011 You are correct. Your receipt no. is 157-6105 Help: Receipt Previous Tries

-Calculate the concentration (M) of free metal ion M+ at V = Ve. (Use the Kf' calculated above)

Just need help with the last problem please and thank you

Explanation / Answer

Titration

Total volume at equivalence point (V = Ve) = 45 + 56.2 = 101.2 ml

at equivalence point all of metal ion forms complex (MY2-) with EDTA

molar concentration [MY2-] = 0.05 M x 45 ml/101.2 ml = 0.022 M

some of it redissolves in solution.

ICE chart

              M+ + EDTA <==> MY2-

I               -           -                0.022

C             +x        +x                  -x

E               x         x               0.022-x

So,

Kf' = [MY2-]/[M+][EDTA]

let x be a small change

8.1 x 10^11 = (0.022)/x^2

x = 1.65 x 10^-7 M

thus,

concentration of free metal ion (M+) at V = Ve would be 1.65 x 10^-7 M

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