What is the formula mass of anhydrous sodium carbonate? (Given that the atomic masses of sodium, carbon and oxygen are 23 u, 12 u and 16 u respectively)
- A.286 u
- B.106 u
- C.83 u
- D.53 u
Correct answer
Explanation
The correct answer is B, 106 u. Anhydrous sodium carbonate is Na₂CO₃, and 2 × 23 + 12 + 3 × 16 = 106 u.
Formula mass is the sum of the atomic masses of all the atoms in one formula unit of an ionic compound. Sodium carbonate has two sodium atoms (46 u), one carbon atom (12 u) and three oxygen atoms (48 u), giving 106 u. The word anhydrous means it holds no water of crystallisation. Washing soda, the crystalline form, is Na₂CO₃·10H₂O; its ten water molecules add 180 u, so its formula mass is 286 u. Sodium carbonate is used to make glass, soap and paper and to remove the permanent hardness of water.
A is wrong: 286 u is the formula mass of washing soda, the decahydrate, not the anhydrous salt.
C is wrong: 83 u comes from counting only one sodium atom, NaCO₃.
D is wrong: 53 u is just half of 106 u and matches no real formula unit.
Exam tip: Na₂CO₃ = 106 u; Na₂CO₃·10H₂O (washing soda) = 286 u.
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