(i) Calculate the mass of 6.023 xx 10^(20) molecules of H_2O. (ii) Calculate the moles of 12g of magnesium. (iii) Calculate the number of molecules present in 50 g of Fe.

(i) Calculate the mass of 6.023 xx 10^(20) molecules of H_2O. (ii) Ca

1.	(i) Calculate the mass of 6.023 xx 10^(20) molecules of H_2O. (ii) Calculate the moles of 12g of magnesium. (iii) Calculate the number of molecules present in 50 g of Fe.
Answer» Solution :Molecular mass of `H_2O = H XX 2 + O xx 1` `= 1 xx 2 + 16 xx 1` `= 2 + 16 = 18` Mass of an ATOM = `("Number of atoms")/("Avagardo.s number") xx "mass of" H_2O` `= (6.023 xx 10^(20))/(6.023 xx 10^(23)) xx 18` `10^(-3) xx 18` Mass of an atom = 0.018 g (II) Number of moles = `("Given mass")/("molecular mass")` `= 12/24 = 1/2 = 0.5` `:. 12 g` MAGNESIUM makes 0.5 moles. (iii) Number of atoms = `("Avagadro number" xx " Given mass)/("Gram atomic mass")` `=(6.023 xx 10^(23) xx 50)/(55.9)` `= 5.387 xx 10^(23)` `:. 50 g Fe` contain `5.387 xx 10^(23)` atoms/molecules.