## Molarity

005113695
Posts: 59
Joined: Fri Sep 28, 2018 12:15 am

### Molarity

How do I find the amount of moles for water molecules in MgSo4 + 7H2O (epsom salts) when there is 5.15g of epsom salt?

Jeremy_Guiman2E
Posts: 82
Joined: Fri Sep 28, 2018 12:29 am

### Re: Molarity

Ok, so we are given 5.15 g of epsom salts, and know the formula unit would be MgSO4 + 7H2O.

We will use the molar mass (246.48 g/mol) to determine how many total moles of epsom salts we have. So, 5.15 g / 246.48 g/mol = 0.0209 mol epsom salts.

However, because there are 7 water molecules in each formula unit, we must multiply by 7. So, 0.0209 mol x 7 = .1463 mol H2O. We will round to 3 significant figures as that was how many our given (5.15 g) had, so it rounds to .146 mol H2O.

Heesu_Kim_1F
Posts: 60
Joined: Fri Sep 28, 2018 12:18 am

### Re: Molarity

The concept that is confusing (even for me at first) and needs to be understood is that in 1 mole of MgSO4+7H2O (epsom salt), there are 7 moles of H2O (water molecules). Thus, you need to first convert the given information 5.15g MgSO4+7H2O into moles of MgSO4+7H2O. Its molar mass is 246.466 g/mol. So, divide 5.15g by 246.466 g/mol, in which you should get 0.0209 mol of MgSO4+7H2O. Then, use stoichiometry, in which you need to multiply 0.0209 mol MgSO4+7H2O by 7 since there are 7 moles of H2O in 1 mole of MgSO4+7H2O. The answer I got was 0.146 mol of H2O. Hope this helps!

005113695
Posts: 59
Joined: Fri Sep 28, 2018 12:15 am

### Re: Molarity

Thank you so much!