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Here is the problem: What is the molar mass of Al2(SO4)3?

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Add the atomic weights of all 17 atoms in one formula unit of Al2(SO4)3.

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Count the atoms of each element. Subscripts multiply everything inside the brackets before them, and a hydrate coefficient multiplies the whole water (or other) unit.

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Look up each atomic weight (g/mol). IUPAC standard atomic weights (abridged, conventional values).

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Multiply each count by its atomic weight.

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Add the contributions.

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Mass percent of each element.

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So the answer is M(Al2(SO4)3) equals 342.132 g/mol.

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The molar mass is the mass of one mole of formula units. It equals the sum of the atomic weights of every atom in the formula, and numerically matches the formula mass in daltons (u).

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Percent composition adds to 100.00 percent.

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Elements: aluminium times 2, sulfur times 3, oxygen times 12.