Determining Number of Moles of Oxygen Atoms

How many moles of oxygen atoms are there in 3.00g of sodium dichromate, Na2CrO7?

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...have been given the mass of our compound (3.00g) so all we need to find the mols is the molar mass. This can be accomplished by adding up the molar masses of the elements involved. Pay careful attention to the subscripts:

Molar mass = Na: (2 x 22.99) + Cr: (2 x 52.00) + O (7 x 16.00) = 261.98 g/mol

Mols = ...