Methane burns completely in oxygen to carbon dioxide and water. How many moles of oxygen are needed per mole of methane?
Answer: A
Balancing gives one methane plus two oxygen to one carbon dioxide plus two water. Failing to balance the hydrogen before the oxygen is what leads to the wrong coefficient here. Two moles of water require two moles of oxygen atoms.
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Consider the following statements about degrees of freedom with reaction.
1. Each independent reaction adds one unknown, its extent.
2. Each independent reaction also allows the species balances to be written.
3. Adding a reaction always makes the problem unsolvable.
Answer: B
Correct answer: 1 and 2 only
Statements 1 and 2 are the two sides of the ledger: each independent reaction adds its extent as an unknown, and it also permits an additional independent species balance to be written. Statement 3 is wrong because those two effects cancel - adding a reaction leaves the degrees of freedom unchanged, so the problem is neither easier nor harder to close.
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An inert species in a reacting system is useful because it
Answer: C
Nitrogen in combustion air is the standard case, and its unchanged quantity relates the inlet and outlet gas flows at once. Its presence dilutes the mixture but does not enter the stoichiometry. Its heat capacity does matter in the energy balance.
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Assertion (A): Atomic balances avoid the need for reaction extents.
Reason (R): Atoms are conserved, so the input equals output form applies to them directly.
Answer: D
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason is the justification. Because atoms are conserved, an element balance takes the simple form input equals output with no generation term, so the extents never enter the equations at all. The trade is interpretive rather than arithmetical: an atomic balance closes the problem without ever telling you how much of each reaction occurred.
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A feed of 100 moles of A undergoes a reaction with 80 per cent conversion to a desired product B and a side product C, with 60 moles of B formed from the reaction of A to B. What is the selectivity in moles of B per mole of C, if 20 moles of C are formed?
Answer: A
Sixty moles of the desired product against twenty of the undesired gives a selectivity of three. Dividing by the feed instead of by the undesired product gives the yield rather than the selectivity. The two are quoted together to describe a reactor fully.
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State whether the following is true or false.
Selectivity and yield are two names for the same quantity.
Answer: B
Correct answer: False
The two answer different questions. Conversion says how much reactant was consumed, selectivity says which product it became, and yield combines them - the wanted product obtained against the theoretical maximum from the feed. A process can convert everything and yield very little if the selectivity is poor, so reporting one of the three alone conveys almost nothing.
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In a combustion calculation the nitrogen entering with the air
Answer: C
The trace nitrogen oxides formed are negligible in a material balance, so the nitrogen is carried through unchanged. That assumption is what makes an Orsat analysis usable for finding the excess air. Nitrogen oxides are counted in emissions, not in the balance.
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