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**Hint:**To find no. of atoms firstly find the number of moles of each element.No. of moles is equal to the ratio of given mass and molar mass. No. of molecules of a substance is equal to the product of no. of moles of given substance and Avogadro number.

**Step by step answer:**1st Case is of 12g of carbon,

Given mass of carbon= 12g

Molar mass of carbon = 12g/mole

And we know that, No. of moles of atom= given mass/molar mass

And by putting values we get, no. of moles of carbon =12/12 = 1 mole

And no. of molecules= no. of moles $ \times $ Avogadro no.

Therefore by putting values,

No. of molecules of carbon= 1 $ \times $ 6.022 $ \times $${10^{23}}$

No. of molecules of carbon = 6.022$ \times $${10^{23}}$ molecules

2nd Case is of 64g of oxygen,

Given mass of oxygen = 64g

Molar mass of oxygen = 16g/mole

And by putting values we get, no. of moles of oxygen =64/16 = 4 moles

Now by putting values,

No. of molecules of oxygen = 4 $ \times $ 6.022 $ \times $${10^{23}}$

No. of molecules of oxygen = 24.088$ \times $${10^{23}}$ molecules

3rd Case is of 72g of water,

Given mass of water = 72g

Molar mass of water = 2$ \times $1 + 16

Molar mass of water =18g/mole

And by putting values we get, no. of moles of water=72/18 = 4 moles

Now by putting values,

No. of molecules of water = 4 $ \times $ 6.022 $ \times $${10^{23}}$

No. of molecules of water = 24.088$ \times $${10^{23}}$ molecules

**Note:**Avogadro's number is the number of units in one mole of any substance (defined as its molecular weight in grams), its value is equal to 6.02214076 × 1023. The units may be electrons, atoms, ions, or molecules, depending on the nature of the substance and the character of the reaction.

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