Which of the following contains the GREATEST number of atoms of argon gas? 5.0 L of Ar at STP, 6.02 x 10^23 atoms of Ar ,O 0.75 moles of Ar, 20.0 g of Ar

Answer 1

So Mary T.,

Convert each value into moles of Ar (It's monatomic, no tricky stuff there). Then compare. You SHOULD know the Universal Gas Law in your sleep by now,

pV = nRT

where p=pressure (such as, in atm); V = volume (such as, in L), n is the # moles of the particular item, R = the Universal Gas Constant (in whatever units you have in the rest of the problem, or else convert each item appropriately!), and T = temperature in Kelvin (note, could also say, degrees Kelvin).

When you finish, you may whistle, Happy Days are Here Again.

(as opposed to, are gone?)

-- Cheers, --Mr. d.

Answer 2

Hi Mary,

First we need to convert each of these values to the same unit: atoms of Ar

5.0 L Ar • (1 mole Ar / 22.4 L) • ((6.022 • 10²³) atoms / 1 mole) = 1.34 • 10²³ Atoms Ar

*Since there’s no direct pathway from Liters to atoms we have steps: Liters → moles → atoms

*Use conversion factors: 1 mole = 22.4 L and 6.022 • 10²³ atoms = 1 mole

*We use Avogadro’s # for other units like: atoms, molecules and particles

6.02 • 10²³ Atoms of Ar

*No need to convert

0.75 moles Ar • ((6.022 • 10²³) atoms / 1 mole) = 4.52 • 10²³ Atoms Ar

*Direct pathway from moles to atoms

*Use conversion factor: 6.022 • 10²³ atoms = 1 mole

20 g Ar • (1 mole Ar / 39.948 g Ar) • ((6.022 • 10²³) atoms / 1 mole) = 3.01 • 10²³ Atoms Ar

*No direct pathway from grams to atoms so: grams → moles → atoms

*Use conversion factors: 1 mole = molar mass of element and 6.022 • 10²³ atoms = 1 mole

Answer: 6.02 • 10²³ Atoms of Ar

Helpful info:

Moles has direct conversions with:

Moles → grams

Moles → Liters

Moles → Atoms, Molecules, or Particles

Moles → Moles (ex: You can convert from moles of Ar to moles of Na)

*Hence why we need multiple conversions sometimes.

*The unit you want to cancel out should be opposite — Ex: Liters is on top and on the conversion factor L is on the bottom. Same with moles. See below example ↓

5.0 L Ar • (1 mole Ar / 22.4 L) • ((6.022 • 10²³) atoms / 1 mole) = 1.34 • 10²³ Atoms Ar

Hope this helps :)