4. for rxn 1, 10.0 ml of a cu2+ solution of unknown concentration was placed in a 250 ml erlenmeyer flask. an excess of ki solution was added. indicator was added and the solution was diluted with h2o to a total volume of 75 ml. for rxn 2, the solution from rxn 1 was titrated with 0.20 m na2s2o3. the equivalence point of the titration was reached when 14.45 ml of na2s2o3 had been added. what is the molar concentration of cu2+ in the original 10.0 ml solution?

0.289 M

Explanation:

In the rxn1, the ions Cu²⁺ will react with the ions I⁻ that comes from KI, and the ions I⁻ will react with themselves:

Cu²⁺ + 4I⁻ → CuI₂ + I₂

Then, the I₂ will react with the ions Na⁺ that comes from Na₂S₂O₃ at the rxn 2:

Na₂S₂O₃ + I₂ → 2NaI + S₂O₃²⁻

The number of moles of Na₂S₂O₃ is the volume (0.01445 L) multiplied by the concentration:

n = 0.01445*0.20

n = 2.89x10⁻³

By the stoichiometry, 1 mol of Na₂S₂O₃ reacts with 1 mol of I₂, so nI₂ = 2.89x10⁻³ mol. And, by the stoichiometry of the other reaction, 1 mol of I₂ is produced with 1 mol of Cu²⁺, thus, nCu²⁺ = 2.89x10⁻³ mol.

The molar concentration is the number of moles divided by the volume is L. Thus, for the original solution with a volume of 10 mL = 0.01 L:

Concentration of Cu²⁺ = 2.89x10⁻³/0.01 = 0.289 M

"among the transition metal elements, manganese and zinc have a slight lowering of electronegativities. the reason behind this is that the nuclear charges of these elements are slightly weaker than the other transition metal elements due to the way their valence electrons are filled up. both manganese and zinc have their valence electrons filled up to the 4th shell with 1 pair of electrons (4s2) completely occupying it. although the electrons in the 3rd shell (3d) are yet to be paired since the outer most shell is filled up, the ability of the nucleus to attract electrons is lessened. hence,   the electronegativity (or the ability to attract electrons) is lessened."