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Here is the problem: How many moles are in 15.8 g of KMnO4?

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Add the atomic weights of all 6 atoms in one formula unit of KMnO4.

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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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Convert the sample mass to moles.

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Number of formula units.

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So the answer is M(KMnO4) equals 158.032 g/mol; 15.8 g equals 0.0999798 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: potassium times 1, manganese times 1, oxygen times 4.