## Exercise E9

Julia Lindner 1I
Posts: 35
Joined: Fri Sep 28, 2018 12:17 am

### Exercise E9

I'm having trouble with exercise E9, about epson salts. I try to calculate the number of atoms of oxygen, mainly by following the method described in example E2, but I keep getting a different answer than the book. I'm also not sure how to calculate the number of formula units. Can anyone help me out?

Anushi Patel 1J
Posts: 61
Joined: Fri Sep 28, 2018 12:19 am
Been upvoted: 1 time

### Re: Exercise E9

I think the tricky part about this problem is the formula for Epsom salt, which is MgS04.7H20 (the "." is supposed to represent the notation for hydrates, but I couldn't figure out how to type it out). The problem states that epsom salt is magnesium sulfate heptahydrate, which means that it has 7 water molecules attached to it that you have to count as part of the molar mass, which turns out to be 246.39 g/mol. To find the number of oxygen atoms, I used dimensional analysis and cancel out units to convert from 5.15 g of epsom salt, to moles of epsom salt, to moles of oxygen (the ratio of moles epsom salt to moles of oxygen is 1:11 since there are 11 oxygen atoms in the formula for epsom salt), to atoms of oxygen (using avogadro's number). Hope that helps!

Posts: 65
Joined: Fri Sep 28, 2018 12:17 am

### Re: Exercise E9

So if something has some prefix-hydrate does that always mean that there is a H20 attached?

Julia Lindner 1I
Posts: 35
Joined: Fri Sep 28, 2018 12:17 am

### Re: Exercise E9

Anushi Patel 1L wrote:I think the tricky part about this problem is the formula for Epsom salt, which is MgS04.7H20 (the "." is supposed to represent the notation for hydrates, but I couldn't figure out how to type it out). The problem states that epsom salt is magnesium sulfate heptahydrate, which means that it has 7 water molecules attached to it that you have to count as part of the molar mass, which turns out to be 246.39 g/mol. To find the number of oxygen atoms, I used dimensional analysis and cancel out units to convert from 5.15 g of epsom salt, to moles of epsom salt, to moles of oxygen (the ratio of moles epsom salt to moles of oxygen is 1:11 since there are 11 oxygen atoms in the formula for epsom salt), to atoms of oxygen (using avogadro's number). Hope that helps!

That does help, thanks!