A party balloon contains 5.50 x 10 ^22 atoms of helium gas. What is the mass, in grams of the helium?

a) 0.365 g He
b) 9.14 x 10^-2 g He
c) 0.365 mol He
d) 9.14 x 10 -2 mol He

Explanation:

1. Convert Atoms to Moles

We must use Avogadro's Number: 6.022*10²³. This is the number of particles (atoms, molecules, ions, etc.) in 1 mole of a substance. In this case, the particles are atoms of helium. We can create a ratio.

Multiply by the given number of helium atoms.

Flip the fraction so the atoms of helium cancel.

2. Convert Moles to Grams

We must use the molar mass, which is found on the Periodic Table.

Use this as a ratio.

Multiply by the number of moles we calculated. The moles will then cancel.

3. Round

The original measurement has 3 significant figures (5, 5, and 0). Our answer must have the same. For the number we calculated, it is thousandth place. The 3 in the ten thousandth place tells us to leave the 5.

The mass is 0.365 grams of helium so choice A is correct.

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