What mass of helium gas (in g) occupies 15.3 L at STP?

Explanation:

We are asked to find the mass of helium gas given a volume.

1 mole of any gas occupies a volume of 22.4 liters. So, 1 mole of helium gas contains 22.4 liters of helium. Create a ratio using this information.

We are converting 15.3 liters of gas to moles, so we multiply by that value.

The units of liters of helium (L He) cancel.

The molar mass is the mass of 1 mole of a substance. These values are found on the Periodic Table as atomic masses, but the units are grams per moles. Look up the molar mass of helium.

Create another ratio using the molar mass.

Multiply by the number of moles we calculated.

The units of moles of helium (mol He) cancel.

The original measurement of volume has 3 significant figures, so our answer must have the same.

For the number we calculated, that is the hundredths place.

The 3 in the thousandths place to the right (2.73391875) tells us to leave the 3 in the hundredths place (2.73391875).

15.3 liters of helium gas is approximately 2.73 grams of helium.

atomic mass = 39

number of neutrons = 20

number of protons= 19

electronic configuration = 1s2 2s2 2p6 3s2 3p6 4s1

the number of electron in first shell = 2

number of electrons in n=3 shell = 8