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Question 1
What does the chemical formula Fe₂O₃ tell us about one unit of this compound?
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Correct answerCorrect!Incorrect
A2 iron atoms and 3 oxygen atoms
Step-by-step walkthrough
Choose a solution method
Method #1Direct Approach
Step 1: Identify the elements and their subscripts
In the formula Fe₂O₃, we have two element symbols: Fe (iron) and O (oxygen). Each has a subscript number written after it.
Step 2: Read the subscripts
The subscript after Fe is 2, meaning there are 2 iron atoms. The subscript after O is 3, meaning there are 3 oxygen atoms.
Step 3: State the answer
One formula unit of Fe₂O₃ contains 2 iron atoms and 3 oxygen atoms. This compound is iron(III) oxide, a common form of rust.
Method #2Process of Elimination
Step 1: Identify what the question asks
We need to correctly read the subscript numbers in Fe₂O₃ to determine how many atoms of each element are present.
Step 2: Eliminate '3 iron atoms and 2 oxygen atoms'
This option reverses the subscripts. The 2 belongs to Fe and the 3 belongs to O, not the other way around.
Step 3: Eliminate '2 iron atoms and 1 oxygen atom'
This option misreads the oxygen subscript as 1. When a subscript of 3 is clearly written after O, there are 3 oxygen atoms, not 1.
Step 4: Eliminate '1 iron atom and 3 oxygen atoms'
This option ignores the subscript 2 after Fe. A subscript of 2 means 2 iron atoms, not 1.
Step 5: Select the correct answer
The only correct reading is 2 iron atoms and 3 oxygen atoms, matching the subscripts in Fe₂O₃ exactly.
Question 2
Which of the following is not a compound?
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Correct answerCorrect!Incorrect
AOzone, O₃
Step-by-step walkthrough
Choose a solution method
Method #1Direct Approach
Step 1: Recall the definition of a compound
A compound contains atoms of two or more different elements bonded together. A substance made of only one element is not a compound.
Step 2: Check each formula for the number of different elements
H₂O contains H and O (two elements ✓), NaCl contains Na and Cl (two elements ✓), CO₂ contains C and O (two elements ✓). But O₃ contains only oxygen — just one element.
Step 3: Identify the non-compound
O₃ (ozone) is an allotrope of oxygen — a different form of the same element. It is not a compound because it contains only one type of element.
Method #2Process of Elimination
Step 1: Identify what we are looking for
We need the substance that is not a compound — meaning it does not contain two or more different elements bonded together.
Step 2: Eliminate H₂O
Water (H₂O) contains hydrogen and oxygen — two different elements — so it is definitely a compound.
Step 3: Eliminate NaCl
Sodium chloride (NaCl) contains sodium and chlorine — two different elements — so it is a compound.
Step 4: Eliminate CO₂
Carbon dioxide (CO₂) contains carbon and oxygen — two different elements — so it is a compound.
Step 5: Select O₃
Ozone (O₃) contains only oxygen atoms. It is an allotrope of oxygen, not a compound. This is the correct answer.

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Step-by-step walkthrough

Step 1: Identify the elements and their subscripts

Step 2: Read the subscripts

Step 3: State the answer

Step 1: Identify what the question asks

Step 2: Eliminate '3 iron atoms and 2 oxygen atoms'

Step 3: Eliminate '2 iron atoms and 1 oxygen atom'

Step 4: Eliminate '1 iron atom and 3 oxygen atoms'

Step 5: Select the correct answer

Step-by-step walkthrough

Step 1: Recall the definition of a compound

Step 2: Check each formula for the number of different elements

Step 3: Identify the non-compound

Step 1: Identify what we are looking for

Step 2: Eliminate H₂O

Step 3: Eliminate NaCl

Step 4: Eliminate CO₂

Step 5: Select O₃

13 more questions in this topic