## Fundamentals E.9

305115396
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Joined: Fri Sep 28, 2018 12:16 am

### Fundamentals E.9

E.9 Epsom salts consist of magnesium sulfate heptahydrate. How many atoms of oxygen are in 5.15 g of Epsom salts?

For this question I know that you have to convert the 5.15 g given to moles in order to get the molar value of MgSO4*7H2O. However I do not understand how you can figure out the atoms of oxygen given this information.

Diane Kang 2F
Posts: 33
Joined: Fri Sep 28, 2018 12:18 am

### Re: Fundamentals E.9

First, convert the given mass 5.15 g MgSO4*7H2O into moles using the molar mass of MgSO4*7H2O, which is 246.483 g/mol:

(5.15 g MgSO4*7H2O) (mol / 246.483g) = 0.02089 mol MgSO4*7H2O

Then, use the molar ratio of MgSO4*7H2O and oxygen, which is (11 mol O / 1 mol MgSO4*7H2O), to find the moles of oxygen:

(0.02089 mol MgSO4*7H2O)(11 mol O / 1 mol MgSO4*7H2O) = 0.229833 mol O

Finally, use Avogadro’s constant to convert moles of oxygen to the number of atoms of oxygen:

(0.229833 mol O)(6.022 x 10^23 atoms / mol) = 1.384 x 10^23 atoms of oxygen

Fayez Kanj
Posts: 62
Joined: Fri Sep 28, 2018 12:18 am
Been upvoted: 1 time

### Re: Fundamentals E.9

Hello!
This is a quick and easy way to do problems like these.

If you multiply the number of moles of molecules by the number of a specific atom in that molecule, you get the number of moles of the specific atom/element. For example, If you have 2 moles of H2SO4, then to get the number of moles of O, you would do 2x4 = 8, since there are 2 moles, and in each mole, there are 4 oxygen atoms, so it totals to 8 moles of oxygen atoms.

In this problem, there are approximately 0.21 moles of magnesium sulfate heptahydrate. Multiply that 0.21 moles by the amount of oxygen atoms there are, which is 11 (7 from the H2O and 4 from the sulfate) to get 0.23 moles of oxygen atoms, which equals 1.38x10^23 atoms of O.

Hope this helps!