Write the equation at the beginning of every problem: mass percent = (mass of chemical/total mass of compound) x 100. Both of the values should be in grams so that they cancel each other out once you solve the equation. The mass of the chemical you’re interested in is the mass given in the problem. If the mass isn’t given, refer to the following section about solving for mass percent when the mass is not given. The total mass of the compound is calculated by summing the masses of all of the chemicals used to make the compound or solution.
Example 1: What is the percent mass of 5g of sodium hydroxide dissolved in 100g of water? The total mass of the compound is the amount of sodium hydroxide plus the amount of water: 100g + 5g for a total mass of 105g. Example 2: What masses of sodium chloride and water are needed to make 175 g of a 15% solution? In this example, you are given the total mass and the percentage you want, but are asked to find the amount of solute to add to the solution. The total mass is 175 g.
Example 1: The mass of the chemical-in-question is 5g of sodium hydroxide. Example 2: For this example, the mass of the chemical-in-question is the unknown you are trying to calculate.
Example 1: mass percent = (mass of chemical/total mass of compound) x 100 = (5 g/105 g) x 100. Example 2: We want to rearrange the mass percent equation to solve for the unknown mass of the chemical: mass of the chemical = (mass percenttotal mass of the compound)/100 = (15175)/100.
Example 1: (5/105) x 100 = 0. 04761 x 100 = 4. 761%. Thus, the mass percent of 5g of sodium hydroxide dissolved in 100g of water is 4. 761%. Example 2: The rearranged equation to solve for mass of the chemical is (mass percenttotal mass of the compound)/100: (15175)/100 = (2625)/100 = 26. 25 grams sodium chloride. The amount of water to be added is simply the total mass minus the mass of the chemical: 175 – 26. 25 = 148. 75 grams water.
Write out the equation at the beginning of every problem: mass percent = (molar mass of element/total molecular mass of compound) x 100. Both values have units of grams per mole (g/mol). This means the units will cancel each other out when you solve the equation. When you aren’t given masses, you can find the mass percent of an element within a compound using molar mass. Example 1: Find the mass percent of Hydrogen in a water molecule. Example 2: Find the mass percent of carbon in a glucose molecule.
Example 1: Write out the chemical formula for water, H2O. Example 2: Write out the chemical formula for glucose, C6H12O6.
Example 1: Look up the molecular weight of Oxygen, 15. 9994; and the molecular weight of Hydrogen, 1. 0079. [9] X Research source Example 2: Look up the molecular weight of Carbon, 12. 0107; Oxygen, 15. 9994; and Hydrogen, 1. 0079.
Example 1: Hydrogen has a subscript of two while oxygen has a subscript of 1. Therefore, multiply the molecular mass of Hydrogen by 2, 1. 00794 X 2 = 2. 01588; and leave the molecular mass of Oxygen as is, 15. 9994 (multiplied by one). Example 2: Carbon has a subscript of 6, hydrogen, 12, and oxygen, 6. Multiplying each element by its subscript gives you: Carbon (12. 01076) = 72. 0642 Hydrogen (1. 0079412) = 12. 09528 Oxygen (15. 9994*6) = 95. 9964
Example 1: Add 2. 01588 g/mol (the mass of two moles of Hydrogen atoms) with 15. 9994 g/mol (the mass of a single mole of Oxygen atoms) and get 18. 01528 g/mol. Example 2: Add all of the calculated molar masses together: Carbon + Hydrogen + Oxygen = 72. 0642 + 12. 09528 + 95. 9964 = 180. 156 g/mol.
Example 1: The mass of hydrogen in the compound is 2. 01588 g/mol (the mass of two moles of hydrogen atoms). Example 2: The mass of carbon in the compound is 72. 0642 g/mol (the mass of six moles of carbon atoms).
Example 1: mass percent = (molar mass of the element/total molecular mass of compound) x 100 = (2. 01588/18. 01528) x 100. Example 2: mass percent = (molar mass of the element/total molecular mass of compound) x 100 = (72. 0642/180. 156) x 100.
Example 1: mass percent = (2. 01588/18. 01528) x 100 = 0. 11189 x 100 = 11. 18%. Thus, the mass percent of Hydrogen atoms in a water molecule is 11. 18%. Example 2: mass percent = (molar mass of the element/total molecular mass of compound) x 100 = (72. 0642/180. 156) x 100 = 0. 4000 x 100 = 40. 00%. Thus, the mass percent of carbon atoms in a molecule of glucose is 40. 00%.
