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### E.9

Posted: **Thu Oct 03, 2019 7:48 pm**

by **April Meza 3K**

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 5.15 g? (c) How many moles of water molecules are in 5.15 g of Epsom salts?

I am not sure how to go about this question, can someone explain? Thanks.

### Re: E.9

Posted: **Thu Oct 03, 2019 8:17 pm**

by **FDeCastro_1B**

formula for magnesium sulfate heptahydrate is MgSO4.7H2O

to do all three problems, you have to find the formula mass of MgSO4.7H2O (which you can do by looking at the periodic table) which is 246.37 g/mol

(a) you need to find the number of moles of the magnesium sulfate heptahydrate. you are given the mass of the sample and in order to find the moles, you have to divide the mass of the sample by the molar mass of magnesium sulfate heptahydrate. after that you have to multiply the moles of the sample to the mole ratio of oxygen because oxygen is what you're asked to focus on. lastly you multiply your previous product with Avogadro's constant to find the number of atoms of oxygen in 5.15g of Epsom salts

(b) this question isn't asking for one specific element so you just divide the mass of the sample by the molar mass of magnesium sulfate heptahydrate then multiply that number of moles to Avogadro's constant to find the formula units

(c) this problem is similar to (a) but instead it's asking for the moles of water molecules. again you divide the mass of the sample by the molar mass of the compound. then you multiply it to the mole ratio of H2O

### Re: E.9

Posted: **Thu Oct 03, 2019 8:57 pm**

by **April Meza 3K**

Thank you! This really helps!