Using Avogadro's Number (6.02*10^23) : Calculate the number of molecules in 3.00 moles H2S. Express your answer numerically in molecules ... Calculate the number of moles of Cl atoms in 1.81*10^24 formula units of magnesium chloride, MgCl2.

Avagadro's number is just a measurement. One mole is 6.022 X 10^23 of anything - atoms, molecules, marbles ... anything.

1) If one mole = 6.022 X 10^23, then 8.00mol of H2S is:

(3.00mol H2S) (6.022 X 10^23 molecules H2S / 1 mol H2S) = 1.8060 X 10^24 molecules H2S.

Rounded to 3 sig figs = 1.81 X 10^24 molecules H2S

part2.

This one uses moles in the stoichiometric sense as well as the measurement. One formula unit of MgCl2 contains 1 mole Mg and 2 moles Cl.

First, figure out how many moles of formula units there are.

(1.81 X 10^24 FU's) (1mol MgCl2 / 6.022 X 10^23 FU's) = 3.0056mol MgCl2.

Now, we know that there are 2 moles of Cl in every mole of MgCl2 (2 Cl atoms in every unit of MgCl2). From this we can determine how many moles of Cl atoms there are:

(3.0056mol MgCl2) (2mol Cl atoms / 1mol MgCl2) = 6.0112mol Cl atoms.

Now round to 3 sig figs = 10.0mol Cl atoms