Question

What are the mole fractions of H3PO4 and water in a solution of 14.5g of H3PO4 in 125g of water?

Answer 1

For `"H"_3"PO"_4`: `0.0209`
For `"H"_2"O"`: `0.9791`

Explanation:

Well let's start with getting moles of both `"H"_3"PO"_4` and `"H"_2"O"` that are present in this sample.

The molecular weight of `"H"_3"PO"_4` is approximately 97.9 g/mol, and the molecular weight of `"H"_2"O"` is approximately 18.02 g/mol. So:

For `"H"_3"PO"_4`:
`(14.5 "g")/(97.9 "g/mol")=0.1481` moles

For `"H"_2"O"`:
`(125 "g")/(18.02 "g/mol") = 6.937` moles

Now we determine the total amount of moles in the sample:
`n_"total"=0.1481+6.937=7.0848` moles

Now to determine mole fractions, we take the individual component moles and divide by the total moles, or in other words:
`n_"i"/n_"total"`

For `"H"_3"PO"_4`:
`0.1481/7.0848 = 0.0209`

For `"H"_2"O"`:
`6.937/7.0848 = 0.9791`

Note that for water we can take `1-"mole fraction of ""H"_3"PO"_4` and get the same answer.

Hope this helps!