A student has 20.0 mL of a solution that is 5.00 times 10^-3 M in Fe^3+, Mg^2+ a

ID: 514520 • Letter: A

Question

A student has 20.0 mL of a solution that is 5.00 times 10^-3 M in Fe^3+, Mg^2+ and Cr^3+ and has a pH of 1.00. To this solution the student adds 10.0 mL of 20% (by mass) NH_4Cl and 5.00 mL of 15 M NH_3. (a) What is the final pH and [OH-] of the buffered solution? (assume that precipitation of metal hydroxides won't significantly effect [OH-] and pH) b) Which of the three metal cations will precipitate as hydroxide salts? c) Can one of the metals be separated from the other two? If so which one and what form is in? d) What are the concentrations of Fe^+3, Mg^+2, and Cr^+3 in the final solution once it reaches equilibrium?

Explanation / Answer

a) POH= PKb+log [salt]/[base]

= 4.74 + log 0.2/15

=4.74+ (-1.87)

=2.86

[OH-]= 1/ANTILOG[2.86]

=13X10-4 M

PH=14-POH=14-2.86=11.14

b) Only Mg2+ can form hydroxide ion in aqueous solution

Mg(OH)2(s)Mg2(aq)+

c) Magnesium can be separated as Mg(OH)2

d) Cannot predict the concentration of metal ions