Question

If a sample of LiF is found to contain 7.73 x 10^24 Li+ ions how many grams of LiF formula units are present?

I tried to work this out and each time i got the incorrect answer. I know the answer is 332.959930255729 as Wiley gave me this however it didn't explain how. Any help would be very appreciated!!!

Answer 1

Think about it in terms of Avogadro's number.

Explanation:

We know that Lithium fluoride is an ionic compound that contains negative fluoride ions and positive lithium ions in a 1:1 ratio.

1 mole of any substance contains `6.022 times 10^23` molecules, and the molar mass for `LiF` is 25.939 `gmol^-1`. The question is how many moles of `LiF` does your amount correspond to?

Divide your number of molecules with Avogadro's number.

`(7.73 times 10^24)/(6.022 times 10^23)=12.836 mol`

As lithium ions exist in a 1:1 ratio, this amount of moles of Lithium ions also correspond to the number of moles of the substance- `LiF`.

Hence to find the mass in grams we multiply the number of moles with the molar mass to find the mass (According to `n=(m)/(M)`)

`n=12.836`
`M=25.939`
`m=?`

`12.836 times 25.939=332.9599` grams

Which is consistent with what Wiley gave you :)