Open in App Suggest Corrections 1 ⚛ percentage composition ⚛ percent by mass (% by mass) ⚛ percent by weight (% by weight) Step 1: Calculate the total mass of each element present in the molecular formula(1) of the compound total mass of element present = number of atoms of this element × relative atomic mass of element(2) Step 2: Calculate the relative molecular mass, Mr, of the compound(3) add together all the total masses for all the elements present in the molecule Step 3: Calculate the percentage compositon : % by mass (or % by weight) of element Please do not block ads on this website. Percent Composition Concepts
Each ball is weighed and it is found that :
What percentage of the total mass of all the balls in the box is due to the black balls?
What percentage of the total mass of all the balls in the box is due to the red ball? 100% by mass = % by mass of black balls + % by mass of red ball 100% = 11% + % by mass of red ball % by mass of red ball = 100 - 11 = 89% Chemists often think of atoms and molecules as really tiny balls, and refer to this as the particle theory of matter. If each ball in the box represents an atom making up a water molecule, H2O, then the diagram below shows a box containing a molecule of water in which
A water molecule, H2O, is made up of 1 oxygen atom and 2 hydrogen atoms. Using the Periodic Table you will find: relative atomic mass (atomic weight) of an oxygen atom is 16 relative atomic mass (atomic weight) of a hydrogen atom is 1.0 What percentage of the total mass of the water molecule is due to hydrogen atoms?
What percentage of the mass of a water molecule is due to the oxygen atom? 100% by mass = % by mass of all hydrogen atoms + % by mass of the oxygen atom 100% = 11% + % by mass of the oxygen atom % by mass of the oxygen atom = 100 - 11 = 89%
Do you know this? Join AUS-e-TUTE! Play the game now! For any element present in any molecule:
And, if you add together the % by mass of every element present in the compound the result will be 100% So, for a compound with the formula XaYbZc we can write mathematical equations (expressions) to find the percentage by mass of elements X, Y and Z in the compound as shown in the table below:
Do you understand this? Join AUS-e-TUTE! Take the test now! Example 1: Calculate the percent by mass (weight) of sodium (Na) and chlorine (Cl) in sodium chloride (NaCl)
The answers above are probably correct if %Na + %Cl = 100%, that is, 39.34% + 60.66% = 100% Example 2 Calculate the percent by mass (weight) of each element present in sodium sulfate (Na2SO4).
The answers above are probably correct if %Na + %S + %O = 100%, that is, 32.37% + 22.58% + 45.05% = 100% Example 3 Calculate the percent by mass (weight) of each element present in ammonium phosphate [(NH4)3PO4]
The answers above are probably correct if : %by mass(N) + %by mass(H) + %by mass(P) + %by mass(O) =100%, that is, 28.19% + 8.11% + 20.77% + 42.93% = 100%
Can you apply this? Join AUS-e-TUTE! Take the exam now! The problem: Jackie the Geologist has just discovered a new iron ore deposit. Jackie took a sample of this iron ore to Chris the Chemist for analysis. First, Chris purified the sample, removing the mud and other impurities from the sample. Then Chris analysed the purified ore sample and found it was composed of 69.94% iron and 30.06% oxygen. What is the most likely formula for the compound that makes up Jackie's ore? Solving the problem: Using the StoPGoPS model for problem solving:
Footnotes: (1) We will be using the term "molecular formula" generically to include ionic compounds as well as covalent compounds. (2) We will be using relative atomic mass of elements which you will find in the Periodic Table but you can use molar mass of the element since the molar mass is just the relative atomic mass of the element expressed in grams. (3) We will be using relative molecular mass (also known as formula mass, molecular weight, or formula weight), but you can use molar mass instead since molar mass is just the relative molecular mass expressed in grams. (4) There may be rounding errors in your calculations so that the result will be extremely close to, but not exactly, 100%. In general you can expect results to be between 99% and 101%. |