What is the molar mass of Pb(SO4)2? Explain how you calculated this value.

Question

Also, how do you figure out the molar mass? My lesson does a really bad job of trying to explain it. 

Answer 1

Hello Athena

 

The molar mass of any element can be looked up from the periodic table.  The molar mass is listed below the chemical symbol for that element.

 

1 mole of any element is equal to its mass expressed in units of grams.  For instance, 1 mole of sodium (Na) contains 22.99 grams; 1 mole of Helium contains 4.00 grams.

 

In Pb(SO₄)₂, you want to first write down the # of each element present in the compound.

 

Pb (lead) = 1

S (sulfur) = 2

O (oxygen) = 8

 

SO₄ (sulfate; i am purposely omitting the ionic charge) is called a polyatomic ion.  As you can see in the formula that SO₄ is written in parentheses followed by a subscript of 2.  This means that there are 2 sulfate ions present in the compound and that you need to double the # of atoms present for Sulfur (1 x 2 = 2) and Oxygen (4 x 2 = 8).

 

Now look up the molar mass of each element from the periodic table and multiply it by the # of atoms present.

 

Pb = 207.20 g/mol x 1 = 207.20 g/mol

 

S = 32.06 g/mol x 2 = 64.12 g/mol

 

O = 16.00 x 8 = 128.00 g/mol

 

Now add up the molar masses of each element from above to get the molar mass of Pb(SO₄)₂ = 207.20 + 64.12 + 128.00 = 399.32 g/mol.