Unit 5 Ch 2: Mass Percent

How do you calculate the mass percent of an element in a compound?

(Mass of element in compound/total mass of the compound) x 100% = mass % of element Ex. Finding the mass percent of Hydrogen in H2O: (2(1.01) amu of Hydrogen / 18.02 amu of H20) x 100% = 11.21% Mass of Hydrogen

How many grams of oxygen can be produced from the decomposition of 100g of KClO3??

1) Find the total mass of the compound 39.10 + 35.45 + 3(16.00) = 122.5 2) Divide the mass of oxygen by the total mass and multiply by 100% (3(16) / 122.55) x 100 = 39.17% 3) What is 39.17% of 100? 39.17 So, you have 39.17 grams of Oxygen ANOTHER EXAMPLE: How much iron can be recovered from 25.0g of Fe2O3? Total mass of compound: 2(55.85) + 3(16.00) = 159.7 Mass percent of iron in the compound: (2(55.45)/159.7) x 100% = 69.94% What is 69.94% of 25? Do 25.0 times 0.6994 17.5g of iron

How do you find the empirical formula of a compound when given the percent of each element in the compound?

Example: 75% Carbon and 25% Hydrogen 1) Assume you have 100g of Compound - Then you'd have 75g of Carbon and 25g of Hydrogen 2) Divide each one by the elements amu (on the periodic table) to find the number of moles of that element - 75/12.01 = 6.2 mol of Carbon - 25/1.01 = 25 mol of Hydrogen 3) Divide each number by the smallest amount - 6.2/6.2 = 1, so 1 atom of Carbon - 25/6.2 = 4.0 (You have to round), so 4 atoms of Hydrogen In this compound, for every carbon atom, there are four atoms of hydrogen, so the formula is CH4 (Methane)

When the empirical formula is given (or enough info is there for you to find it) and the molecular mass is also given, how do you find the molecular formula of a compound?

Example: The compound (empirical formula) is NO2. It's molecular mass is 92 g/mol: 1. Find the mass of the compound (in this case, NO2) - 14.01g + 2(16.00)g = 46.01 g/mol 2. Divide the molecular mass that is given by the mass of the compound to see what number you should multiply the amount of atoms in the empirical formula by - 92/46.01 = 2, so twice the number of atoms of each element in NO2 - NO2 become N2O4

percent composition

the percent by mass of each element in a compound