Chapter 16
Q. 16.4
Coulometric Titration
A 2.000-mL volume containing 0.611 3 mg of cyclohexene/mL is to be titrated in Figure 16-8. How much time is required for titration at a constant current of 4.825 mA?

Step-by-Step
Verified Solution
The moles of cyclohexene are
\frac{(200 mL)(0.611 3 mg/mL)}{(82.146 mg/mmol)} = 0.014 88 mmol
2Br^{−} → Br_{2} + 2e^{−} (16-10)
In Reactions 16-10 and 16-11, each mole of cyclohexene requires 1 mol of Br_{2}, which requires 2 mol of electrons. For 0.014 88 mmol of cyclohexene to react, 0.029 76 mmol of electrons must flow. From Equation 16-3,
Moles of e^{−} = \frac{I · t}{F} ⇒ t = \frac{(moles of e^{−})F}{I}
t = \frac{(0.029 76 × 10^{−3} mol)(96 485 C/mol)}{(4.825 × 10^{−3} C/s)} = 595.1 s
It will require just under 10 min to complete the reaction.
Test Yourself How much time is required to titrate 1.000 mg of cyclohexene at 4.000 mA? (Answer: 587.3 s)