m (O2) = 64 g
n (H2O) -?
1) Find the amount of substance O2:
n (O2) = m (O2) / Mr (O2) = 64/32 = 2 mol;
2) Find the amount of substance O in O2 (by definition):
n (O in O2) = n (O2) * 2 = 2 * 2 = 4 mol;
3) Find the amount of substance O in H2O (by definition):
Let the amount of substance H2O be 1 mol, then
n (O in H2O) = n (H2O) = 1 mol;
4) Find the amount of substance O in H2O:
Because in О2 the amount of substance О is early 4 mol, then in Н2O 4 mol of О is needed. Then
n (H2O) = n (O in O2) = 4 mol.
Answer: The amount of the substance H2O, which contains as many O atoms as they are in O2, is 4 mol.
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