Which of the following is the formulas unit mass of NaCl?
58.5 u
Formula Unit Mass Calculation for NaCl
The formula unit mass of an ionic compound like NaCl (Sodium Chloride) represents the sum of the average atomic masses of all the atoms in its simplest chemical formula unit. For NaCl, the formula unit consists of one sodium (Na) atom and one chlorine (Cl) atom.
Step 1: Identify Components and Atomic Masses
To calculate the formula unit mass of NaCl, we first identify the constituent elements and find their average atomic masses using a periodic table:
- Sodium (Na): The average atomic mass is approximately \(22.99 \, \text{u}\).
- Chlorine (Cl): The average atomic mass is approximately \(35.45 \, \text{u}\).
Step 2: Calculate the Formula Unit Mass
The formula unit mass is the sum of the atomic masses of the atoms in the formula unit. For NaCl, the calculation is:
\( \text{Formula Unit Mass of NaCl} = (\text{Atomic Mass of Na}) + (\text{Atomic Mass of Cl}) \)
Plugging in the values:
\( \text{Formula Unit Mass of NaCl} = 22.99 \, \text{u} + 35.45 \, \text{u} \)
\( \text{Formula Unit Mass of NaCl} = 58.44 \, \text{u} \)
This value, \(58.44 \, \text{u}\), is commonly rounded to \(58.5 \, \text{u}\) in many contexts.
Step 3: Compare with Given Options
We compare our calculated value with the options provided:
| Option Number | Given Value | Match? |
| 1 | \(71 \, \text{u}\) | No |
| 2 | \(58.5 \, \text{u}\) | Yes |
| 3 | \(46.5 \, \text{u}\) | No |
| 4 | \(94 \, \text{u}\) | No |
The calculated formula unit mass, when rounded, aligns with the value presented in Option 2.
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