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A solution of sodium sulfate contains 92 g of Na+ ions per kilogram of water. The molality of Na+ ions in that solution in mol kg⁻¹ is:

1. 8
2. 4
3. 12
4. 16

Correct answer: 4

Solution

Molality = moles of Na+ per kg of water = (92 g)/(23 g/mol) / 1 kg = 4 mol/kg.

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