How to Calculate Mass Percentage

Mass percent indicates the percentage of each element in a chemical compound. ^{[1] X Research Source} To find mass percent, we must know the mass mp of the elements in the compound in grams/mp or the number of grams of the substances forming the solution. ^{[2] X Research Source} Mass percent is calculated using a simple formula that divides the mass of the element (or solute) by the mass of the compound (or solution).

Table of Contents

Calculate mass percent when mass is known

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Determine the equation for the mass percent in the mixture. The basic formula for mass percent in a mixture is: mass percent = (mass of substance/mass of mixture) x 100. You must eventually multiply by 100 to get the percentage value. ^{[3] X Research Sources}

Write the equation at the beginning of solving the problem: mass percent = (mass of substance/mass of mixture)x 100 .
The mass of the substance will be given in the problem. If the problem does not provide, please refer to the section below on how to find the mass percent when the mass is unknown.
The mass of a mixture is equal to the sum of the masses of the substances that make up the mixture or solution.

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Calculate the mass of the mixture. Once you know the masses of the elements or compounds, simply add them up to get the mass of the final mixture or solution. This is the denominator in the mass percent formula. ^{[4] X Research Sources}

Example 1: Calculate the mass percent of 5g sodium hydroxide when dissolved in 100g water?
- The mass of the mixture is the sum of the masses of sodium hydroxide and water: 100g + 5g. So the mass of the mixture is 105g.
Example 2: Calculate the mass of sodium chloride and water needed to make 175g of a solution of 15% concentration?
- In this example, you know the mass of the mixture and the mass percentage, the problem asks to find the mass of solute added. The mass of the mixture is 175 g.

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Determine the mass of the substance to find the mass percent. When the problem asks you to find the “mass percentage” of a substance, you have to find the mass of the substance as a percentage of the sum of the masses of all the components. Write down the mass of the substance to find the mass percent. This is the numerator in the mass percent formula. ^{[5] X Research Sources}

Example 1: Mass of sodium hydroxide (substance to find mass percent) is 5g.
Example 2: In this example, the mass of the substance needs to find the unknown mass percent, and you are looking for it.

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Substitute the variables into the mass percent equation. Once you’ve determined the value of each variable, you can simply plug them into the equation.

Example 1: mass percent = (mass of substance/mass of mixture) x 100 = (5 g/105 g) x 100.
Example 2: We need to convert the mass percent equation to calculate the mass of the unknown substance: mass of substance = (mass percent * mass of mixture)/100 = (15*175)/100.

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Calculate mass percent. Now that the equation is filled in, you just need to calculate the mass percent. Divide the mass of the substance by the mass of the mixture, and then multiply it by 100. This is the mass percent of the substance in the mixture.

Example 1: (5/105) x 100 = 0.04761 x 100 = 4.761%. Therefore, the mass percent of 5g of sodium hydroxide dissolved in 100g of water is 4.761%.
Example 2: The equation after conversion to calculate the mass of a substance is (mass percent*mixture mass)/100: (15*175)/100 = (2625)/100 = 26.25 grams of sodium chloride .
- The mass of water added is the mass of the mixture minus the mass of the substance: 175 – 26.25 = 148.75 grams of water.

Calculate mass percent when mass is unknown

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Determine the equation for the mass percent of the compound. The basic formula for calculating mass percent in a compound is: mass percent = (mass mp element/molecular mass of the compound) x 100. Mass mp element is the mass of one mp element while molecular mass is the mass of one mp of compound. ^{[6] X Research Source} You must eventually multiply by 100 to get a percentage value. ^{[7] X Research Sources}

Write the equation at the beginning of solving the problem: mass percent = (mp mass of element/molecular mass of compound) x 100 .
The units of these two values are grams per mp (g/mp).
When the problem does not give mass, you can use the mass mp to calculate the mass percent of the element.
Example 1: Calculate the mass percent of hydrogen in a water molecule.
Example 2: Calculate the mass percent of carbon in a glucose molecule.

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Write chemical formula . If the problem does not give the chemical formulas of each compound, you need to write them down. If the problem is for a chemical formula, skip this step and go to the “Find the mass of each element” step.

Example 1: Write the chemical formula of water, H ₂ O.
Example 2: Write the chemical formula of glucose, C ₆ H ₁₂ O ₆ .

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Find the mass of each element in the compound. Look up the molecular weight of each element in the chemical formula on the periodic table. Elemental masses are usually written below the chemical symbol. Write down the mass of each element in the compound. ^{[8] X Research Sources}

Example 1: We find that the atomic mass of oxygen is 15.9994; and the atomic mass of hydrogen is 1.0079. ^{[9] X Research Source}
Example 2: We find that the atomic mass of carbon is 12.0107; oxygen is 15.9994; and hydrogen is 1.0079.

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Multiply the atomic mass by the ratio mp. Determine the number of mp (mp ratio) of each element in the chemical compound. The ratio mp is given as the lower number in the chemical formula of the compound. Multiply the atomic mass of each element by the ratio mp. ^{[10] X Research Source}

Example 1: Hydrogen has a subscript of two while oxygen has a subscript of 1. So multiply the molecular mass of hydrogen by 2, 1.00794 X 2 = 2.01588; and the molecular mass of oxygen is 15.9994 (multiply by one).
Example 2: Carbon has a subscript of 6, hydrogen is 12, and oxygen is 6. Multiply the atomic mass of each element by the subscript, and we get:
- Carbon (12.0107*6) = 72.0642
- Hydrogen (1.00794*12) = 12,09528
- Oxygen (15.9994*6) = 95.9964

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Calculate the total mass of the compound. Add the masses of all the elements in the compound. You can calculate the total mass of a compound through the mp-ratio masses. This number will be the denominator in the mass percent equation. ^{[11] X Research Source}

Example 1: Add 2.01588 g/mp (the mass of two mp of hydrogen atoms) with 15.9994 g/mp (the mass of one mp of oxygen atom) to get 18,01528 g/mp.
Example 2: Add all the masses together: carbon + hydrogen + oxygen = 72.0642 + 12,09528 + 95.9964 = 180.156 g/mp.

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Determine the mass of the element to calculate the mass percent. When the problem asks you to find “mass percent,” you have to find the mass of a particular element in a compound as a percentage of the sum of the masses of all the elements. Determine and write down the mass of that element. This mass is the mass in mp ratio. This number is the numerator in the mass percent equation. ^{[12] X Research Source}

Example 1: The mass of hydrogen in the compound is 2.01588 g/mp (the mass of two mp of hydrogen atoms).
Example 2: The mass of carbon in the compound is 72.0642 g/mp (the mass of six mp of carbon atoms).

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Substitute the variables into the mass percent equation. Once you’ve determined the values of each variable, simply plug them into the equation determined in the first step: mass percent = (mass mp element/molecular mass of the compound) x 100 .

Example 1: mass percent = (mass mp element/molecular mass of the compound) x 100 = (2.1588/18.1528) x 100.
Example 2: mass percent = (mass mp of element/molecular mass of compound) x 100 = (72.0642/180,156) x 100.

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Calculate mass percent. Now that the equation is filled in, you just need to calculate the mass percent. Divide the mass of the element by the total mass of the compound, and then multiply by 100. This is the mass percent of the element in the compound.

Example 1: mass percent = (2.01588/18.01528) x 100 = 0.11189 x 100 = 11.18%. So, the mass percent of the hydrogen atom in the water molecule is 11.18%.
Example 2: mass percent = (mass mp of element/molecular mass of compound) x 100 = (72.0642/180,156) x 100 = 0.4000 x 100 = 40.00%. So, the mass percent of the carbon atom in the glucose molecule is 40.00%.

Steps

Calculate mass percent when mass is known

Calculate mass percent when mass is unknown

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