Question

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

Answer 1

`\boxed {\boxed {\sf 2.73 \ g \ He}}`

Step-by-step explanation:

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

1. Convert Liters to Moles

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.

`\frac {1 \ mol \ He }{ 22.4 \ L \ He}`

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

`15.3 \ L \ He *\frac {1 \ mol \ He }{ 22.4 \ L \ He}`

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

`15.3 *\frac {1 \ mol \ He }{ 22.4}`

`\frac {15.3}{22.4} \ mol \ He`

`0.6830357143 \ mol \ He`

2. Convert Moles to Grams

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.

• He: 4.0026 g/mol

Create another ratio using the molar mass.

`\frac {4.0026 \ g \ He}{1 \ mol \ He}`

Multiply by the number of moles we calculated.

`0.6830357143 \ mol \ He *\frac {4.0026 \ g \ He}{1 \ mol \ He}`

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

`0.6830357143*\frac {4.0026 \ g \ He}{1 }`

`2.73391875 \ g \ He`

3. Round

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).

`2.73 \ g \ He`

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