How to Calculate Mass Percentage: 13 Steps (with Pictures)

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How to Calculate Mass Percentage: 13 Steps (with Pictures)
How to Calculate Mass Percentage: 13 Steps (with Pictures)

Video: How to Calculate Mass Percentage: 13 Steps (with Pictures)

Video: How to Calculate Mass Percentage: 13 Steps (with Pictures)
Video: How to Calculate Mass Percent of a Solution 2024, April
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Mass percent is the percentage of each element in a chemical compound. To find the mass percent, you need the molar mass of the element in the compound in grams/mol, or the mass in grams used to make the solution. The mass percent can be calculated easily using the basic formula by dividing the mass of the element (or solute) by the mass of the compound (or solution).

Step

Method 1 of 2: Determining Mass Percent If the Mass of the Substance is Known

Calculate Mass Percent Step 1
Calculate Mass Percent Step 1

Step 1. Determine the mass percent equation for a substance

The basic formula for mass percent of a substance is: mass percent = (mass of substance/total mass of compound) x 100. You must multiply the result of the division by 100 at the end of the calculation to express the value in percent.

  • Write the equation at the beginning of each question: mass percent = (mass of substance/total mass of compound) x 100.
  • The mass of the chemical you are looking for is the known mass value in the problem. If not given, read the next section to solve problems that do not provide data on the mass of the substance.
  • The total mass of a compound is calculated by adding up the masses of all the substances that make up the compound or solution.
Calculate Mass Percent Step 2
Calculate Mass Percent Step 2

Step 2. Calculate the total mass of the compound

If you know all the masses of the constituent elements or compounds, you only need to add them to get the total mass of the compound or solution. This number will be the divisor in the mass percent calculation.

  • Example 1: What is the mass percent of 5 g of sodium hydroxide dissolved in 100 g of water?

    The total mass of the compound is the mass of sodium hydroxide plus the mass of water, ie: 100 g + 5 g = 105 g

  • Example 2: What mass of sodium chloride is needed to make 175 g of a 15% solution?

    In this example, the total mass and mass percent are given in the problem, but you are asked to find the amount of solute that needs to be added to a solution with a total mass of 175 g

Calculate Mass Percent Step 3
Calculate Mass Percent Step 3

Step 3. Find the mass of the substance in question

When asked to find "mass percent", you must find the mass of a particular chemical (which is asked in the problem) as a percentage compared to the total mass of all its constituents. Write down the mass of the chemical in question. This mass will be the numerator of the mass percent calculation.

  • Example 1: the mass of the substance in question is 5 g of sodium hydroxide.
  • Example 2: in this example, the mass of the chemical you are looking for is unknown and you must calculate it.
Calculate Mass Percent Step 4
Calculate Mass Percent Step 4

Step 4. Plug the known variables into the mass percent equation

After determining the value of each variable, plug them all into the equation.

  • Example 1: mass percent = (mass of chemical substance/total mass of compound) x 100 = (5 g/105 g) x 100.
  • Example 2: in this problem, we need to change the writing of the mass percent equation to find the mass of the unknown chemical to be: mass of chemical substance = (mass percent*total mass of compound)/100 = (15*175)/100.
Calculate Mass Percent Step 5
Calculate Mass Percent Step 5

Step 5. Calculate the mass percent

Now that the equation is numerically populated, all you need to do is complete the mass percent calculation. Divide the mass of the chemical by the total mass of the compound, then multiply by 100. This calculation will give you the percent mass of the substance.

  • Example 1: (5/105) x 100 = 0.04761 x 100 = 4.761%. Thus, the mass percent of 5 g of sodium hydroxide dissolved in 100 g of water is 4.761%.

  • Example 2: Change the writing of the equation to calculate the mass of the chemical substance to (percent mass*total mass of the compound)/100: (15*175)/100 = (2625)/100 = 26.25 grams of sodium chloride.

    The amount of water that must be added is the total mass minus the mass of the substance, namely: 175 – 26, 25 = 148, 75 grams of water

Method 2 of 2: Solving the Mass Percentage Problem If the Mass of a Substance is Unknown

Calculate Mass Percent Step 6
Calculate Mass Percent Step 6

Step 1. Determine the mass percent of an element

The basic formula for mass percent of an element is: mass percent = (molar mass of an element/total molecular mass of a compound) x 100. The molar mass of a substance is the mass of one mole of the element, while the molecular mass is the mass of 1 mole of a compound as a whole. You must multiply the result by 100 at the end of the calculation to express the value in percent.

  • Write the equation at the beginning of each question: mass percent = (molar mass of an element/total molecular mass of the compound) x 100.
  • Both of these numbers are expressed in grams per mole (g/mol).
  • When the mass of a substance is not given in the problem, the mass percent of an element in a compound can be found using its molar mass.
  • Example 1: Find the mass percent of hydrogen in one water molecule.
  • Example 2: Find the mass percent of carbon in one glucose molecule.
Calculate Mass Percent Step 7
Calculate Mass Percent Step 7

Step 2. Write down the chemical formula

If the chemical formula for the compound is not given in the problem, you must write it down. However, if given in the problem, you can skip this step, and move on to the "find the mass of each element" step.

  • Example 1: write the chemical formula for water, H2O.
  • Example 2: write the chemical formula for glucose, C6H12O6.
Calculate Mass Percent Step 8
Calculate Mass Percent Step 8

Step 3. Find the mass of each element in the compound

Find the atomic weight of each element in the chemical formula on the periodic table. The mass of an element can usually be found under its chemical symbol. Write down the mass of each element in the compound.

  • Example 1: find the atomic weight of oxygen (15.9994), and the atomic weight of hydrogen (1.0079).
  • Example 2: find the atomic weights of carbon (12, 0107), oxygen (15, 9994), and hydrogen (1, 0079).
Calculate Mass Percent Step 9
Calculate Mass Percent Step 9

Step 4. Multiply the mass by the mole ratio of the compound

Find out how many moles (mole ratio) each element is in a chemical compound. The mole ratio is expressed in small numbers in the compound. Multiply the atomic mass of each element by its mole ratio.

  • Example 1: in a water molecule, there are 2 hydrogen atoms and 1 oxygen atom. So, multiply the atomic mass of hydrogen by 2 (1,00794 X 2 = 2,01588), and let the mass of oxygen remain at 15,9994 (because it is multiplied by the number 1).

  • Example 2: in a glucose molecule, there are 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. Multiply each element by its atomic number:

    • Carbon (12, 0107*6) = 72, 0642
    • Hydrogen (1, 00794*12) = 12, 09528
    • Oxygen (15, 9994*6) = 95, 9964
Calculate Mass Percent Step 10
Calculate Mass Percent Step 10

Step 5. Calculate the total mass of the compound

Add up the total mass of all the elements that make up the compound. Using the mass calculated using the mole ratio, you can find out the total mass of the compound. This number will be the divisor in the mass percent equation.

  • Example 1: add 2,01588 g/mol (the mass of 2 moles of hydrogen atoms) by 15,9994 g/mol (the mass of one mole of oxygen atoms) to 18.01528 g/mol.
  • Example 2: add up all calculated molar masses: carbon + hydrogen + oxygen = 72, 0642 + 12, 09528 + 95, 9964 = 180, 156 g/mol.
Calculate Mass Percent Step 11
Calculate Mass Percent Step 11

Step 6. Determine the mass of the element in question

When asked to find "mass percent," you'll need to find the mass of a particular element in a compound expressed as a percentage of the total mass of all the elements that make it up. Determine the mass of the element in question and write it down. This mass is the mass calculated using the mole ratio. This number will be the numerator of the mass percent equation.

  • Example 1: the mass of hydrogen in the compound is 2.01588 g/mol (mass of 2 moles of hydrogen atoms).
  • Example 2: the mass of carbon in the compound is 72.0642 g/mol (mass of 6 moles of carbon atoms).
Calculate Mass Percent Step 12
Calculate Mass Percent Step 12

Step 7. Plug the variables into the mass percent equation

After getting the value of each variable, plug it into the equation written in the first step: mass percent = (molar mass of the element/total molecular mass of the compound) x 100.

  • Example 1: mass percent = (molar mass of the element/total molecular mass of the compound) x 100 = (2,01588/18, 01528) x 100.
  • Example 2: mass percent = (molar mass of the element/total molecular mass of the compound) x 100 = (72, 0642/180, 156) x 100.
Calculate Mass Percent Step 13
Calculate Mass Percent Step 13

Step 8. Calculate the mass percent

Now that the equation is complete, all you need to do is complete the mass percent calculation. Divide the mass of the element by the total mass of the compound, then multiply by 100. The result is the mass percent of the element.

  • 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 the compound) x 100 = (72, 0642/180, 156) x 100 = 0, 4000 x 100 = 40, 00%. Thus, the mass percent of carbon atoms in a glucose molecule is 40.00%.

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