Sally has constructed a concentration cell to measure Ksp for MCln. She construc

Question

Sally has constructed a concentration cell to measure Ksp for MCln. She constructs the cell by adding 2 mL of 0.05 M M(NO3)n to one compartment of the microwell plate. She then makes a solution of MCln by adding KCl to M(NO3)n. She adds 7.861 mL of the resulting mixture to a second compartment of the microwell plate. Sally knows n (the charge on the metal ion) = +3 She has already calculated [Mn+] in the prepared MCln solution using the Nernst equation. [Mn+] = 6.639 M How many moles of [Cl-] must be dissolved in that compartment? _____ mol (Hint: start by finding how many moles of [Mn+] are dissolved in that compartment of the microwell plate) Provide your response to three digits after the decimal.

Explanation / Answer

Write the balanced chemical equation for the formation of MCln as

Mn+ (aq) + n Cl- (aq) --------> MCln

It is known that n = +3 and hence, the formula of the chloride is MCl3 (chloride has oxidation number -1). Thus, as per the above equation,

1 mole M3+ = 3 moles Cl-.

Analysis of 7.861 mL of MCln gave [Mn+] = 6.639 M; therefore, moles of Mn+ in the well = (volume of Mn+ in L)*(molarity of Mn+) = (7.861 mL)*(1 L/1000 mL)*(6.639 M) = 0.052189 mole.

Moles of Cl- that must be added = (0.052189 mole Mn+)*(3 moles Cl-/1 mole Mn+) = 0.15657 mole ? 0.156 mole (correct to three decimal places, ans).

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