# Between each of the given pairs, which has the greater number of particles (atoms, molecules, or ions)? Show your solution. a. 27 g aluminum, Al or 4 g helium, He b. 2 moles sodium chloride, NaCl or 18 g water, H20

The two substances in a, 27 grams of aluminum and 4 grams of helium, have the same number of particles. Both are elements, so their representative particles are atoms. You can find the number moles of atoms by the dividing the mass of each element by its molar mass. The given quantity of each substance is equal to its molar mass and therefore there is one mole of each:

Aluminum: (27 grams)/(27 grams/mole) = 1 mole = 6.02 x10^23 atoms

Helium: (4 grams)(4 grams per mole) = 1 mole = 6.02 X 10^23 atoms

Beween the two compounds in b, there are more representative particles of sodium chloride than of water:

Sodium chloride:

(2 moles NaCl)( 6.02 X 10^23 molecules/mole) = 1.204 x 10^24 molecules NaCl

(It's more correct to refer to particles of NaCl as formula units since it exists as a crystal lattice and not an individual molecule.)

Water:

(18 grams H2O)(18 grams/mole) = 1 mole = 6.02 X 10^23 molecules

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