Current & Electrochemical Stoichiometry

In voltaic/galvanic cells electrical current is created, while in an electrolytic cell electrical current is consumed to drive the reaction. 

The Ampere

The ampere or amp represents the SI unit for electrical current and represents charge per second. 

Example: Gold can be plated out of a solution containing Au3+ based on the following half reaction:

Au3+ (aq) + 3 e -  ---->  Au (s)

a) What mass of gold is plated by a 41 minute flow of 6.8 A current?

4m

Example: A solution of Mn+5 is used to plate out Mn in an electrochemical cell. If a total of 1.13 g of Mn is plated out in a total time of 1600 seconds, what was the electrical current used? (MW of Mn is 54.94 g/mol)

3m

Example: If steady current of 15 amperes is provided by a stable voltage of 12 Volts for 600 seconds, answer each of the following questions.

a)  Calculate the total charge that passes through the circuit in this time.

b)  Calculate the total number of moles of electrons that pass through the circuit in this time.

c)  Calculate the total amount of energy that passes through the circuit in this time.

d)  Calculate the power that the battery provides during this process.

 

 

5m

Current & Electrochemical Stoichiometry Additional Practice Problems

A metal forms the salt MCl 3. Electrolysis of the molten salt with a current of 0.700 A for 6.63 h produced 3.00 g of the metal. What is the molar mass of the metal?

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What mass of platinum could be plated on an electrode from the electrolysis of a Pt(NO3)2 solution with a current of 0.500 A for 55.0 s?

a) 27.8 mg

b) 45.5 mg

c) 53.6 mg

d) 91.0 mg

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A current of 5.00 A is passed through an aqueous solution of chromium (III) nitrate for 30.0 min. How many grams of chromium metal will be deposited at the cathode?

a) 0.027 g

b) 1.62 g

c) 4.85 g

d) 6.33 g

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A 215 mL sample of a 0.500 M NaCl solution with an initial pH of 7.00 is subjected to electrolysis. After 15.0 minutes, a 10.0 mL portion (or aliquot) of the solution was removed from the cell and titrated with 0.100 M HCI solution. The endpoint in the titration was reached upon addition of 22.8 mL of HCl. Assuming constant current, what was the current (in A) running through the cell?

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A current of 11.3 A is applied to 1.25 L of a solution of 0.552 M HBr converting some of the H+ to H2(g), which bubbles out of solution. What is the pH of the solution after 73 minutes?

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A 0.0251 L sample of a solution of Cu + requires 0.0322 L of 0.129 M KMnO4 solution to reach the equivalence point. The products of the reaction are Cu2+ and Mn2+. What is the concentration of the Cu2+ solution?

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A sample of impure tin of mass 0.535 g is dissolved in strong acid to give a solution of Sn2+. The solution is then titrated with a 0.0448 M solution of NO 3-, which is reduced to NO(g). The equivalence point is reached upon the addition of 0.0344 L of the NO3- solution. Find the- percent by mass of tin in the original sample, assuming that it contains no other reducing agents.

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A metal forms the fluoride MF 3. Electrolysis of the molten fluoride by a current of 3.86 A for 16.2 minutes deposits 1.25 g of the metal. Calculate the molar mass of the metal.

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The molar mass of a metal (M) is 50.9 g/mol; it forms a chloride of unknown composition. Electrolysis of a sample of the molten chloride with a current of 6.42 A for 23.6 minutes produces 1.20 g of M at the cathode. Determine the empirical formula of the chloride.

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What mass of aluminum metal can be produced per hour in the electrolysis of a molten aluminum salt by a current of 25 A?

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A major source of sodium metal is the electrolysis of molten sodium chloride. What magnitude of current produces 1.0 kg of sodium metal in 1 hour?

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Silver can be electroplated at the cathode of an electrolysis cell by the half-reaction:

Ag+ (aq) + e– → Ag (s)

What mass of silver would plate onto the cathode if a current of 6.8 A flowed through the cell for 72 min?

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Copper can be electroplated at the cathode of an electrolysis cell by the half-reaction:

Cu2+(aq) + 2 e → Cu(s)

How much time would it take for 325 mg of copper to be plated at a current of 5.6 A?

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What mass of lead sulfate is formed in a lead-acid storage battery when 1.00 g of Pb undergoes oxidation?

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Determine the optimum mass ratio of Zn to MnO 2 in an alkaline battery.

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The following half-reactions combine to make an electrolytic cell:

K+ (aq) + e- → K (s)                                       E° = −2.93

Li+ (aq) + e- → Li (s)                                      E° = −3.05

If a current of 1.5 Amps is placed on this cell for 5 minutes, which of the following describes the mass of solid formed (in grams) and the voltage required of the current.

[1 Amp = 1 coul/s;    96500 coul = 1 mol e-]

A.  0.0324 g;  - 0.12 V            

B. 0.000539 g ; - 0.12 V                     

C. 0.00466 g;  0.12 V                   

D. 0.0324 g;  5.98 V                          

E.  0.00466 g;  5.98 V

 

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If the following half-reactions are used in an electrolytic cell:

            Li+(aq) + e  → Li(s)                E° = −3.04

            Mg2+(aq) + 2 e  → Mg(s)       E° = −2.38

And 10 Amps are applied for 1 minute, determine which solid would form and how much would form in grams.

A.  0.043 grams of Lithium

B.  0.022 grams of Lithium

C.  0.15 grams of Magnesium

D.  0.076 grams of Magnesium

E.  0.086 grams of Lithium

 

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Amps are equivalent to which unit?

A.  Volts

B.  Moles of e

C.  Coulombs

D.  Volts / second

E.  Coulombs / second

 

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How many seconds will be required to deposit 6.23 g of Ag from a solution that contains Ag+ ions if a current of 0.35 A is used?

a. 1.7 x 106

b. 1.6 x 104

c. 2.0 x 103

d. 2.1 x 102

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Electric current, or flow of electrons, is measured in Amperes (A). One Ampere is the delivery of one coulomb (C) of charge per second. What mass of Zinc (in g) is oxidized (to Zn 2+) by a dry cell battery that supplies 125 mA of current for two hours (recall that Faraday's constant is the charge in coulombs on a mole of electrons)?

 

a. 0.003

b. 0.1

c. 0.3

d. 1.0

e. 3.0

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A solution of Mn+5 is used to plate out Mn in an electrochemical cell. If a total of 3.70 g of Mn is plated out in a total time of 1 seconds, what was the electrical current used? (MW of Mn is 54.94 g/mol)

 

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Gold can be plated out of a solution containing Au 3+ based on the following half reaction:

Au 3+ (aq) + 3 e -      →       Au (s)

 

What mass of gold is plated by a 53 minute flow of 8.2 A current?

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