***Always round percent by mass to the hundredths ... to form compound XY, what is the percent by mass of X ... Example. Carbon reacts with oxygen to ...
Law of Definite Proportions and Law of Multiple Proportions
Law of Definite Proportions: A given compound always contains elements in a certain proportion by mass. (Constant composition). Atoms combine in whole number ratios, so their proportion by mass will always be the same.
Example H2O is always made up of 2 atoms of H and one atom of O. The ratio of O to H in water is always 16:2 or 8:1.
Percent by mass % by mass = mass of element mass of compound
x 100
***Always round percent by mass to the hundredths
Example A 20.00 g sample of sucrose contains 1.30 g of hydrogen. What is the mass percent of hydrogen in the sucrose? % by mass = mass of element x 100 mass of compound % Hydrogen = 1.30g x 100 20.00 % Hydrogen = 6.50%
Example If 3.5 grams of X reacts with 10.5 grams of Y to form compound XY, what is the percent by mass of X in the compound? First use the law of conservation of mass Massr = massp Mass x + Mass y = Mass xy 3.5g + 10.5g = Mass xy Mass of xy = 14.0 g Second use % by mass = mass of element x 100 mass of compound % X = mass of X x 100 mass of XY % X = 3.5g x 100 14.0 g %X = 25.00 %
Law of Multiple Proportions When the same two elements combine to form more than one compound: the ratios of the mass of one element in the first compound to its mass in the second compound, (as it combines with the same mass of the other element), can always be expressed as ratios of small whole numbers( ex: 1:3 or 2:5).
Example Carbon reacts with oxygen to form two compounds as shown: Compound A: 2.41 g C, 3.22 g O. Compound B: 6.71 g C, 17.9 g O. Find the lowest whole number ratio of C that react with an equal mass of O.
1. Find the grams of carbon per 1 g of oxygen for each compound. Compound A 2.41g/3.22 g =0.748g/1g Compound B 6.71g/17.9 g = 0.375g/1g
2. Divide the highest answer by the smallest answer. 0.748/0.375 = 1.9946 = 2 (ratio is 2:1)
Lead forms two compounds with oxygen as shown: Compound A: 2.98 g Pb, 0.461 g O. Compound B: 9.89 g Pb, 0.763 g O. For a given mass of oxygen, what is the lowest whole number mass ratio of lead in the two compounds?
