5. A 5.0 mL volume of 0.002M SCN- is mixed with 5.0 mL of 0.02M Fe 3+ to form th

Question

5. A 5.0 mL volume of 0.002M SCN- is mixed with 5.0 mL of 0.02M Fe 3+ to form the blood-red complex FeNCS 2+. The equilibrium molar concentration of the FeNCS 2+ determined from a calibration curve, is 7.0 x 10-4 mol/L. Calculate, in sequence, each of the following quantities in the aqueous solution to determine the equilibrium constant for the reaction. (Note: The calculations correspond to Part C of the data table). Show work for all calculations below. (Hint: Look at equations 34.6, 34.7 and 34.8) Fe3+ (aq) + SCN – (aq) FeNCS 2+

(a) Moles of FeNCS 2+that form in reaching equilibrium

(b) Moles of Fe 3+ that react to form the FeNCS 2+ at equilibrium

(c) Moles of SCN – that react to form the FeNCS 2+ at equilibrium

(d) Moles of Fe 3+ initially placed in the reaction system

(e) Moles of SCN – initially placed in the reaction system

(f) Moles of Fe 3+ (unreacted) at equilibrium

(g) Moles of SCN – (unreacted) at equilibrium

(h) Molar concentration of Fe 3+ (unreacted) at equilibrium

(i) Molar concentration of SCN – (unreacted) at equilibrium

(j) Molar concentration FeNCS 2+ at equilibrium _____________________ work

(k) Kc = [FeNCS 2+] [Fe 3+][ SCN –] _____________________ work

Explanation / Answer

5. Reaction,

Fe3+ + SCN- <==> FeSCN^2+

(a) moles of FeNCS^2+ at equilibrium = 7 x 10^-4 M x 0.01 L = 7 x 10^-6 mol

(b) moles of Fe3+ that react to form FeNCS2+ at Eq. = 7 x 10^-6 mol

(c) moles of SCN- react to form FeNCS2+ at Eq. = 7 x 10^-6 mol

(d) moles of Fe3+ initially placed in reactor = 0.02 M x 5 ml/1000 = 1 x 10^-4 mol

(e) moles of SCN- initially placed in reactor = 0.002 M x 5 ml/1000 = 1 x 10^-5 mol

(f) moles of Fe3+ (unreacted) at Eq. = 1 x 10^-4 - 7 x 10^-6 = 9.3 x 10^-5 mol

(g) moles of SCN- (unreacted) at Eq. = 1 x 10^-5 - 7 x 10^-6 = 3 x 10^-6 mol

(h) Molar concentration of Fe3+ (unreacted) at Eq = 9.3 x 10^-5 mol/0.01 L = 9.3 x 10^-3 M

(i) molar concentration of SCN- (unreacted) at Eq = 3 x 10^-6 mol/0.01 L = 3 x 10^-4 M

(j) Molar concentration of FeNCS2+ at Eq. = 7 x 10^-4 M

(k) Kc = 7 x 10^-4/(9.3 x 10^-3 x 3 x 10^-4) = 251 (or 250.896 unrounded value)

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