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Aluminium sulfate, Al2(SO4)3, has a formula mass of 342.14 amu. How many moles are in 78.452 g of it?
A2.293 mol
B4.361 mol
C0.2293 mol
D0.4586 mol
Answer & Solution
Correct answer: C. 0.2293 mol
1. A formula mass of 342.14 amu means a molar mass of 342.14 g/mol.
2. Moles equal mass divided by molar mass.
3. Substituting gives 78.452 g / 342.14 g/mol = 0.2293 mol.
4. Sanity check: 78 g is a bit under a quarter of 342 g, so the answer sits just under 0.25 mol.
5. The value 4.361 mol inverts the division and reports molar mass over mass.
_Source: OpenStax Chemistry (CC BY 4.0), Ch 3 "Composition of Substances and Solutions", section 3.1 Formula Mass and the Mole Concept_
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