A sample of calcium oxide (CaO) has a mass of 2.80 g. The molar mass of CaO is 56.08 g/mol. How many moles of CaO does this sample contain?

Use moles = mass ÷ molar mass. For this sample, 2.80 g ÷ 56.08 g/mol = 0.0499 mol (to three significant figures). Therefore, the sample contains 0.0499 moles of CaO, choice A.

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