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Which of the following is the formula unit mass of NaCl?
71 u
46.5 u
94 u
58.5 u
58.5 u
The formula unit mass of a substance is the sum of the atomic masses of all atoms present in a formula unit of that compound. For sodium chloride (NaCl), it is calculated by adding the atomic mass of sodium (Na, 23 u) and chlorine (Cl, 35.5 u), giving 58.5 u.
The formula unit mass of a substance is the sum of the atomic masses of all atoms present in a formula unit of that compound. For sodium chloride (NaCl), it is calculated by adding the atomic mass of sodium (Na, 23 u) and chlorine (Cl, 35.5 u), giving 58.5 u.
Just like calculating the total bill of a grocery basket by adding the individual prices of each item (Sodium + Chlorine), the formula unit mass is the total atomic 'price' of all ions in the ionic lattice formula unit.
Remember: Na is 23, Cl is 35.5, add them up for 58.5!
Formula unit mass=∑(Atomic masses of all atoms in formula unit)
Atomic mass of Na=23 u
Atomic mass of Cl=35.5 u
Formula unit mass of NaCl=Atomic mass of Na+Atomic mass of ClFormula unit mass of NaCl=23 u+35.5 u=58.5 u
Ionic compounds like NaCl do not exist as independent molecular entities; hence the term 'formula unit mass' is used instead of 'molecular mass'.
The unit 'u' stands for unified atomic mass unit.
Sodium chloride exists in a face-centered cubic crystal lattice structure.
Used in stoichiometry and calculating molar masses for chemical reactions.
Essential in determining solution concentrations (molarity, molality).
Option A (71 u) corresponds to the molecular mass of chlorine gas (Cl2).
Option B (46.5 u) is an arbitrary incorrect distractor.
Option C (94 u) is another distractor.
Option D (58.5 u) is the correct calculated formula unit mass of sodium chloride (NaCl).
D is correct — The formula unit mass of sodium chloride (NaCl) is 58.5 u, obtained by summing the atomic masses of sodium (23 u) and chlorine (35.5 u).
Always remember that ionic compounds are represented by formula units rather than molecules, making 'formula unit mass' the correct terminology in place of molecular mass.
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