Apentix/AP Chemistry/Unit 4: Chemical Reactions/Question
AP Chemistryeasymcq1 pt

In the expression 2H₂O, what does each numeral indicate, and which one may be changed while balancing an equation?

A.The coefficient 2 counts formula units and may change; the subscript 2 is fixed by the compound's identity
B.Both numerals may be adjusted until the atom tallies match
C.The leading 2 counts H atoms per molecule; the subscript counts molecules
D.Neither numeral affects the number of atoms taking part

Explanation

Core Concept

A coefficient multiplies the entire formula and states how many molecules or formula units participate, so it is freely adjustable during balancing. The subscript fixes how many atoms sit inside one molecule, which defines what the substance is. Raising water's subscript to force an atom tally would invent H₂O₂, hydrogen peroxide, a different compound entirely, so subscripts stay untouched.

Correct Answer

AThe coefficient 2 counts formula units and may change; the subscript 2 is fixed by the compound's identity

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