## Finding number of atoms, formula units and moles

Odalis Trasvina 1A
Posts: 22
Joined: Fri Sep 25, 2015 3:00 am

### Finding number of atoms, formula units and moles

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?

ssendhil
Posts: 27
Joined: Fri Sep 25, 2015 3:00 am

### Re: Finding number of atoms, formula units and moles

In regards to writing the formula, we know that magnesium has a charge of 1+ and sulfate, SO4, has a charge of 1-. Sulfate is a polyatomic ion so the charge of the whole thing is (SO4)1-. When you combine them you end up with MgSO4 and the "heptahydrate" is seven water molecules. ("Hepta" meaning 7 and "hydrate" implying water). It's formula therefore is MgSO4•7H2O . Be careful to not forget the hydrate when calculating the molar mass for the other parts of the problem. Good luck!

Kelsey Ouyang 3H
Posts: 27
Joined: Fri Sep 25, 2015 3:00 am

### Re: Finding number of atoms, formula units and moles

The formula of Magnesium Sulfate Heptahydrate is MgSO4$\cdot$7H20 (Magnesium ion has a +2 charge, Sulfate has a -2 charge and heptahydrate is 7H2O since the prefix hepta indicates 7)
a) We first start by finding the formula mass of the magnesium sulfate heptahydrate which is 246.48 g/mol
To find the atoms of O we need to find how many moles of the magnesium heptahydrate from the sample there is so we divide the amount of grams of the sample by the molar mass. After finding the amount of moles of the sample, we will then multiply by the mole ratio to find the number of moles of oxygen and then lastly we multiply by Avogadro's constant to find out how many oxygen atoms we have
atoms of O=(515 g/ (246.48 g/mol))(11 mol O atoms/1 mol MgSO4$\cdot$7H2O)(6.022x10^23 atoms/mol) = 1.38 x 10^23
b) To find the formula units of the compound we just need to take the amount of sample and divide it by the molar mass to find out how many moles of the sample we have. Then we use Avogadro's constant to find out how many formula units we have of the compound.
formula units = (5.15g/(246.48 g/mol))(6.022x10^23)
c)To determine the moles of water molecules in 5.15 g of Epsom salts, we just need to find out how many moles of the sample there is and multiply it by the mole ratio.
moles of H2O= (5.15 g/ (246.48 g/mol))(7 mol H2O/mol MgSO4$\cdot$7H2O) = 0.146 mol H2O

Maggie Eberhardt - 2H
Posts: 97
Joined: Sat Jul 20, 2019 12:16 am

### Re: Finding number of atoms, formula units and moles

Kelsey Ouyang 3H wrote:The formula of Magnesium Sulfate Heptahydrate is MgSO4$\cdot$7H20 (Magnesium ion has a +2 charge, Sulfate has a -2 charge and heptahydrate is 7H2O since the prefix hepta indicates 7)
a) We first start by finding the formula mass of the magnesium sulfate heptahydrate which is 246.48 g/mol
To find the atoms of O we need to find how many moles of the magnesium heptahydrate from the sample there is so we divide the amount of grams of the sample by the molar mass. After finding the amount of moles of the sample, we will then multiply by the mole ratio to find the number of moles of oxygen and then lastly we multiply by Avogadro's constant to find out how many oxygen atoms we have
atoms of O=(515 g/ (246.48 g/mol))(11 mol O atoms/1 mol MgSO4$\cdot$7H2O)(6.022x10^23 atoms/mol) = 1.38 x 10^23
b) To find the formula units of the compound we just need to take the amount of sample and divide it by the molar mass to find out how many moles of the sample we have. Then we use Avogadro's constant to find out how many formula units we have of the compound.
formula units = (5.15g/(246.48 g/mol))(6.022x10^23)
c)To determine the moles of water molecules in 5.15 g of Epsom salts, we just need to find out how many moles of the sample there is and multiply it by the mole ratio.
moles of H2O= (5.15 g/ (246.48 g/mol))(7 mol H2O/mol MgSO4$\cdot$7H2O) = 0.146 mol H2O

Why do we multiply by Avogadro's constant in part b?

Cassidy Kohlenberger 3D
Posts: 51
Joined: Wed Sep 18, 2019 12:19 am

### Re: Finding number of atoms, formula units and moles

Regarding part b, we multiply by Avogadro's number to convert the moles we have into formula units. Since formula units are representative of molecules, we use Avogadro's number in the conversion. 1 mole = 6.022 x 10^23 molecules, atoms, formula units

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