chemistry question

Question

A gas mixture called Heliox, 6.11% O₂ and 93.89% He by mass, is used in scuba tanks for descents more than 65 m below the surface. Calculate the mole fractions of He and O₂ in this mixture.

mole fraction of He?

mole fraction of O_2?

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Answer 1

If you’re not sure what mole fraction is let’s first define it:

Mole fraction = M_f A = Moles A / Total moles (A + B + C +...all the moles present)

You might’ve sensed that we need to get to moles of He and O₂. Your senses are right! How do we get there? We only have “percentages by mass?”

To “deal with this,” let’s assume there are 100g of total gas - that is, of both O₂ and He.

If there are 100g total of O₂ and He let’s figure out how many grams we have (which then can help us out figure out the number of MOLES).

We multiply 100g x 6.11% O₂ = 6.11g of O₂

We multiply 100g x 93.89% He = 93.89 g of He.

Ok, so what do we do now?

Now, we convert each gas to moles.

O₂ = 6.11g / 32.0g = 0.191 mol O₂

He = 93.89g / 4 = 23.5 mol He

So, what do we do now?

Let’s go back to the definition of mole fraction above and plug-in our values:

Mole fraction = M_f A = Moles A / Total moles (A + B + C +...all the moles present)

M_f O₂ = 0.191 mol O₂ / Total moles (0.191 mol O₂ + 23.5 mol He)

0.191 O₂ / 23.7 total moles (O₂ + He)

0.008 M_f O₂

Now for He:

M_f He = 23.5 mol He / 23.7 total moles

M_f He = 0.992