A student weighs out 8.70 g of manganese dioxide (MnO2). How many moles of manganese atoms are present in this sample? (Molar masses: Mn = 54.9 g/mol, O = 16.0 g/mol)
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Correct answer
A. 0.100 mol
Principle or equation
The amount of substance in moles is calculated by dividing the mass of the sample by its molar mass. For a compound, the molar mass is the sum of the atomic masses of its constituent atoms. The number of moles of a particular element in a compound is equal to the number of moles of the compound multiplied by the number of atoms of that element per formula unit.
Why this answer is correct
First, calculate the molar mass of MnO2: 54.9 + 2(16.0) = 86.9 g/mol. Then, moles of MnO2 = 8.70 g / 86.9 g/mol = 0.100 mol. Since each formula unit of MnO2 contains one Mn atom, the moles of Mn atoms are also 0.100 mol.
Example
For example, to find moles of oxygen atoms in 0.200 mol of CO2, multiply 0.200 by 2 to get 0.400 mol O atoms.
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