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2 Answers

From the formula Al(NO3)3 we understand that there are 9 oxygen atoms in one Al(NO3)3 compound.
if 1mol → 9 mol
Then , 5.25103 mol → 9 5.25*103 mol
One mole is 6.022*1023(Avogadro's number). So multiply it with Avogadro'a number to get your answer.

One unit of Al(NO3)3 has 9 O atoms. There are 3.16 x 10+21 of the said unit in the given mol. Multiply this by 9 to get the number of O atoms in 5.25 x 103 mol, which is 2.85 x 10+22.
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