Which of the following samples contains 2.0 * 1023 atoms ?

a) 8.0 g O2 b) 3.0 g Be c) 8.0 g C d) 19.0 g F2

Molar atomic mass of Be=9

Number of atoms in 3g Be= (Avogadro number )(mass of Be)/ molar mass of Be

=(6.023 x1023)(3g)/9g = 2.0 x1023

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Ans: b) 3.0g of Be

Atomic Mass of Be = 9 g

No. of moles = Given Mass / Molar Mass

= 3 / 9 = 1 / 3 Moles

No. of moles = No. of particles / Avogadro's Constant

1 / 3 = No of atoms / 6.022 * 1023

= 6.022 * 1023 / 3 = No. of atoms 

No.of atoms = 2.0 * 1023

Answer: b) 3.0 g of Be

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