Question

Calculate the number of grams of oxygen in 0.10 mol of Na₂CO₃.10H₂O.

Answer 1

1 mole of Na₂CO₃.10H₂O contains 13 moles of oxygen atoms. 

0.1 mole Na₂CO₃.10H₂O contains 13 × 0.1 = 1.3 moles of oxygen atoms. 

Mass of oxygen atoms = 1.3 moles × 16g/mole = 20.8 g.

Comments

the ans is actually 20.8 g
Na2CO3.10H2O has 13 *16 grams of oxygen in 1 mole = 208grams
therefore in 0.1 mole it will be = 208*0.1 = 20.8g

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why we have use 16 not anything else

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16 is the mass of 1 mole of oxygen and it is constant for one mole.