How many hydrogen atoms are present in a 2.65 gram sample of hydrazine (N2H4) ?

4.95*10^22 atoms of hydrogen.

Explanation:

Now, we must remember that the number of elementary entities in one mole of a substance (atoms, molecules, ions, etc) is given by the Avogadro's number. Avogadro's number is numerically equal to 6.02*10^23.

We must first find the number of moles of hydrazine corresponding to 2.65 g

We find that from;

Number of moles = mass of hydrazine given / molar mass of hydrazine

Molar mass of hydrazine = 32.0452 g/mol

Number of moles of hydrazine = 2.65/32.0452 g/mol = 0.0823 moles

If 1 mole of hydrazine contains 6.02*10^23 hydrogen atoms

0.0823 moles of hydrazine will contain 0.0823 moles * 6.02*10^23 = 4.95*10^22 atoms of hydrogen.