find molar mass

Question

find molar mass of a1₂(so₄)₃

Answer 1

To find the molar mass of the entire compound you want to add together the atomic masses of all of the atoms in the chemical formula. On this one that means taking

2*Al + 3*S + 12*O

=2(26.98) + 3(32.07) + 12(16)

=53.96 + 96.21 + 192

=342.17

meaning that the molar mass here is 342.17 g/mol

Answer 2

Okay, molar masses are pretty simple to calculate.  

 

First, you calculate how many atoms of each type are in the formula.

 

I believe that is aluminum in the beginning -

 

Al₂ indicates 2 atoms of aluminum.  

 

inside the parenthesis, you have a sulfate group consisting of one atom of Sulfur S  and 4 atoms of Oxygen O.

 

For each molecule, there are two atoms of Aluminum and three sulfate groups.  

 

That means there will be 2 atoms of Aluminum, 3 atoms of Sulfur and 12 atoms of Oxygen. 

 

One mole of atoms of an element weighs, in grams, the atomic weight of that atom.  SO, you'll read the atomic weight of each of those three kinds of atoms, and multiply by the number of atoms in a molecule, and then add them all together.

 

For instance, the weight of just the Oxygen in one mole of Al₂(S0₄)₃ is 12 times the molecular weight of Oxygen, which is 15.9994.  That's going to be roughly 192. (Use a calculator).

 

Just as a quick estimate, Aluminum is around 27 * 2 = 54 and Sulfur is around 32 * 3 = 96, so the whole mole of Aluminum tri-Sulfate is going to end up in the ballpark of 342.  

 

Use a periodic table or google to get the exact molecular weights, multiply by the right numbers, and add them.  You can use my ballpark estimate above to check your answer - it should be within a gram or two of 342 grams.