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### E9

Posted: **Thu Oct 05, 2017 3:53 pm**

by **Shannon Wasley 2J**

The question is:

Epsom salts consist of magnesium sulfate heptahydrate. Write its formula. (a) How many atoms of oxygen are in 5.15 g of Epsom salts? (b) How many formula units of the compound are present in 5.15g? (c) How many moles of water molecules are in 5.15 g of Epsom salts?

For part a, I understand that we divide the 5.15 grams by the molar mass of the entire compound, but why do we multiply that by "11" moles of O atoms?

### Re: E9 [ENDORSED]

Posted: **Thu Oct 05, 2017 4:27 pm**

by **Alex Leve 3F**

After dividing the 5.15 grams of magnesium sulfate heptahydrate by it's molar mass, we get the number of moles of magnesium sulfate heptahydrate present. However, the question asks how many oxygen atoms are present, so we must use the stoichiometric ratio of moles of oxygen per one mole of MgSO4.7H2O. From the formula, we see that there are 11 moles of oxygen (4 in the magnesium sulfate part plus 7 in the hydrate) per mole of MgSO4.7H2O.

Hope this helps!

### Re: E9

Posted: **Thu Oct 05, 2017 4:29 pm**

by **Nickolas Manipud 1C**

You multiply it by 11 O atoms because that is how many O atoms are in magnesium sulfate heptahydrate (MgSO4 * 7H20)

Multiplying it by 11 will give you moles of O which you then multiply by Avagadro's constant (6.022 * 10^23) to get the number of O atoms.

Hope this helped!

### Re: E9

Posted: **Thu Oct 05, 2017 5:21 pm**

by **Kelly Kiremidjian 1C**

dividing by 246.48 g.mol^-1 gives the molar mass of the entire thing, and it just asks for oxygen so you multiply by 11 because there are 11 moles of Oxygen in the equation. Then after you convert to atoms

### Re: E9

Posted: **Fri Oct 06, 2017 5:31 pm**

by **Lorie Seuylemezian-2K**

Can someone explain step by step how to do step B

### Re: E9

Posted: **Sat Oct 07, 2017 3:37 pm**

by **Shannon Wasley 2J**

Step 1: Since we know are calculating for formula units, we want to convert our grams of the compound to moles so we can use Avogadro's number which is 6.022 x 10^23 formula units/ mol

5.15g MgSO4 7H20 x 1 mol MgSO4 7H2O/ 246.48gMgSO4 7H2O (this is the molar mass of the compound). The grams would cancel out, leaving 2.09 x 10^-2 moles of MgSO4 7H2O

Step 2: Now that we have calculated the moles of the compound, we need to get the formula units of the compound, so we multiply by Avogadro's number

2.09 x 10^-2 moles x 6.022 x 10^23 formula units/ 1 mol. The moles cancel out leaving, 1.26 x 10^22 formula units of the compound, MgSO4 7H2O.