Calculating Percentage Composition Calculate the percentage of carbon, hydrogen, and oxygen (by mass) in C12H22O11. Calculate the percentage of potassium, by mass, in K2PtCI6.

Answer: Step 1: To calculate percent by mass, you need t etermine two things: the mass of just the element, and the molar mass of the whole compound. Then, you take the molar mass of just the element and divide it by the molar mass of the whole compound, and multiply by 100%. % composition of the element=(number of atoms of element)(atomic weight of element)/(formula weight of substance).*100%.