Calculating Formula Weights for Common Chemical Compounds

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In summary, the conversation involved a question about the formula weights of three compounds: potassium phosphate, iron(III) oxide, and diphosphorous pentasulfide. The formula weight for potassium phosphate was calculated using the atomic masses of K, S, and O, resulting in a value of 212.3 amu. However, the individual asking for help did not understand how this value was obtained. The conversation also mentioned the formula weight for iron(III) oxide and diphosphorous pentasulfide, as well as a potential formula for the latter compound that includes two moles of water ions.
  • #1
courtrigrad
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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!
 
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  • #2
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.
 
  • #3
The rest corresponds to two moles of water ions, as 2*18=36 grams/mol. So the total formula should be like K2SO4.2H2O.
 
Last edited:

FAQ: Calculating Formula Weights for Common Chemical Compounds

What is the formula weight of a chemical compound?

The formula weight of a chemical compound is the sum of the atomic weights of all the elements present in the compound. It is expressed in atomic mass units (amu) or grams per mole (g/mol).

How is the formula weight calculated?

To calculate the formula weight, you need to first determine the molecular formula of the compound. Then, you can find the atomic weights of each element from the periodic table and multiply them by the number of atoms present in the compound. Finally, add all the atomic weights together to get the formula weight.

Why is it important to calculate the formula weight of a compound?

Calculating the formula weight is important in various scientific fields such as chemistry and pharmacology. It helps in determining the molecular structure and properties of a compound, as well as in accurately measuring and preparing solutions.

Can the formula weight be used to convert between mass and moles?

Yes, the formula weight can be used to convert between mass and moles. By using the formula weight, you can convert the mass of a substance in grams to moles by dividing it by the formula weight. Similarly, you can convert moles to mass by multiplying the number of moles by the formula weight.

Are there any common mistakes to avoid when calculating formula weights?

Yes, there are some common mistakes to avoid when calculating formula weights. These include using the wrong atomic weights, forgetting to multiply the atomic weights by the number of atoms, and rounding off the atomic weights too early in the calculation. It is important to double-check your calculations and use accurate atomic weights to avoid these errors.

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