Calculate the number of grams of each element...

Question

Calculate the number of grams of each element in 100 g of phenolphthalein if the compound is 75.46% C, 4.43% H, and 20.10% O by mass. Convert each to moles.

Answer 1

The mass of each element in 100 g of phenolphthalein is its percentage in g: mass of C = 75.46 g; mass of H = 4.43; mass of O = 20.10 g.

To calculate the moles of each element you divide its mass by its molar mass (mass of 1 mole in g/mol which has the same value than its atomic mass given in periodic table): moles of C = 75.46/12.001 = 6.287 mol; moles of H = 4.43/1.008 = 4.40 mol; moles of O = 20.10/16.00=1.256 mol).

This method can help you to determine the empirical formula of phenolphthalein by dividing the moles of each element by the smallest number of moles from them:

C = 6.287/1.256 = 5; H = 4.40/1.256 = 3.50 = 7/2; and O = 1.256/1.256 = 1; we need all of them whole numbers; to delete the fraction we multiply all the numbers by 2: C10H7O2 is the empirical formula of phenolphthalein.   

Answer 2

Simply divide the percentage by 100% and then multiply by the mass of the molecule:

 

(mass of element) = [(percent of element)/100%] x (mass of molecule)