Calculating Formula Weights for Common Chemical Compounds

[courtrigrad]

Hello all

I encountered the following problems:

What is the formula weight of:

a. potassium phosphate

b. iron(III) oxide

c. diphosphorous pentasulfide

I know that the formula for (a) is K2S04. Hence I know there are 2K's 1 S and 4 O's. After obtaining the atomic masses, I sum them all up. The answer is 212.3 amu but I don't get this.

Any help would be appreciated

Thanks!

 

[wolfson_1123]

K=39.098
S=32.060
O=15.999

So:

K2=78.197
S=32.060
O4=63.998

So:

K2SO4=174.254

Do the same for the other ones.

 

[chem_tr]

The rest corresponds to two moles of water ions, as 2*18=36 grams/mol. So the total formula should be like K₂SO₄.2H₂O.