Question

A reactor is supplied with 12 moles of C, and it undergoes following reactions C + 0.5O2 -> CO and C + O2 -> CO2, if 4 moles of CO2 was produced and 4 moles of C remains unreacted, what are the number of moles of CO produced?

(a) 1

(b) 2

(c) 3

(d) 4

The question was posed to me in an online interview.

My query is from Processes Involving Multiple Units topic in section The Chemical Reaction Equation & Stoichometry, Material Balances for Processes Involving Reaction and Multiple Units of Basic Chemical Engineering

Answer 1

To solve this, we need to analyze the material balance and the stoichiometry of the reactions involved.

Given:

• 12 moles of C (carbon) are supplied to the reactor.
• Two reactions occur:
  • Reaction 1: C + 0.5O₂ → CO
  • Reaction 2: C + O₂ → CO₂
• 4 moles of CO₂ are produced.
• 4 moles of C remain unreacted.

We need to find the number of moles of CO produced.

Step 1: Set up the material balances

Let:

• x be the moles of C that react according to Reaction 1 (C + 0.5O₂ → CO).
• y be the moles of C that react according to Reaction 2 (C + O₂ → CO₂).

Step 2: Write the equations

• The total moles of C reacting:

  $$x+y=12-4=8(\text{because 4 moles of C remain unreacted})$$

• The moles of CO₂ produced are given as 4, and each mole of C reacting with O₂ produces 1 mole of CO₂, so:

  $$y=4$$

• Now substitute $y=4$ into the first equation:

  $$x+4=8\Rightarrow x=4$$

Step 3: Calculate the moles of CO produced

From Reaction 1, each mole of C produces 1 mole of CO. Since $x=4$ moles of C react to form CO, the number of moles of CO produced is:

4 moles of CO.

Final Answer:

The number of moles of CO produced is:

(d) 4.