How many oxygen atoms are there in 7.00 g of sodium dichromate, na2cr2o7?

From the periodic table:
molar mass of sodium = 23 gm
molar mass of Cr = 51.99 gm
molar mass of oxygen = 16 gm

molar mass of Na2Cr2O7 = 2(23) + 2(51.99) + 7(16) = 261.98 gm

number of moles in 7 gm = mass / molar mass = 7 / 261.98 = 0.0267 moles

1 mole of  Na2Cr2O7  contains 7 moles of oxygen, therefore:
number of moles of oxygen in 0.0267 moles = 0.0267 x 7 = 0.1869 moles

To find the number of atoms, we multiply the number of moles by Avogadro's number as follow:
number of atoms = 0.1869 x 6.02 x 10^23 = 1.125138 x 10^23 atoms