Mass Percent: Study with Video Lessons, Practice Problems & Examples

Mass percent, commonly referred to as weight percent, is a crucial concept in chemistry that quantifies the proportion of a specific element within a compound. It is calculated by dividing the mass of the element in grams by the total mass of the compound in grams, and then multiplying the result by 100. This calculation is essential for determining the percent composition of an element in a compound, aiding in the analysis of a substance's chemical makeup. Understanding mass percent is fundamental for tasks such as stoichiometry calculations, which are central to chemical reactions and product formation.

Mass Percent Concept

Here we're going to say that mass percent, also known as weight percent, is a percentage of a given element in a compound. Now we're going to say here it deals with the molar mass N grams of the selected element and one mole of the compound.

So this translates to say that mass percent equals the mass or grams of an element divided by the grams of the compound and then multiplying that by 100. Using this helps us to determine the percent composition of any particular element within a given compound.

Mass   percent = Mass   of   element Mass   of   compound   ×   100

Mass Percent

It says determine the weight percent of carbon found in sucrose. Alright. So here weight percent, mass percent, the same thing. So the mass percent of carbon would be grams of carbon divided by grams of sucrose times 100. If we look at this sucrose, we have 12 carbons, 22 hydrogens, and 11 oxygens. Multiply them by their atomic masses from the periodic table. So when we do that, we get 144.12 grams of carbon, 22.176 grams of hydrogen, and then we get 176 grams of oxygen. We take this amount, which is the amount of carbon, plug it in, and then we're going to add these three totals to get the complete mass of sucrose, which is 342.296 grams. So that number goes here. So divide those two numbers and multiplying by 100 gives us 42.1 percent carbon. So, this would be the weight percent of carbon within sucrose.

Determine the percent composition of nitrogen and oxygen with nitrogen dioxide, NO₂.

• A. 25.0% N and 75.0% O
• B. 15.5% N and 84.5% O
• C. 30.4% N and 69.6% O
• D. 69.6% N and 30.4% O

Here’s what students ask on this topic:

How do you find mass percent?

To find the mass percent (also known as mass percentage or percent composition) of an element in a compound, you'll need to follow these steps:

1. Determine the molar mass of the entire compound by adding up the atomic masses of all the atoms in the compound. You can find atomic masses on the periodic table.
2. Identify the molar mass of the element in question within the compound. If the element appears more than once in the compound, multiply its atomic mass by the number of times it occurs.
3. Divide the molar mass of the element by the molar mass of the entire compound. This will give you a decimal.
4. Multiply the decimal by 100 to convert it into a percentage. This is the mass percent of the element in the compound.

For example, if you're calculating the mass percent of oxygen in water (H₂O), you'd find the molar mass of water (about 18.02 g/mol), the molar mass of oxygen in the compound (about 16.00 g/mol), divide 16.00 g/mol by 18.02 g/mol to get approximately 0.888, and then multiply by 100 to get 88.8% as the mass percent of oxygen in water.

How can I find percent composition by mass?

To find the percent composition by mass of a compound, you'll need to follow these steps:

1. Obtain the molecular formula of the compound, which tells you the number of atoms of each element present in one molecule.
2. Use the periodic table to find the atomic mass of each element in the compound.
3. Multiply the atomic mass of each element by the number of atoms of that element in the molecule to get the total mass of each element in one molecule of the compound.
4. Add up all the total masses of the elements to find the molecular mass of the compound.
5. Calculate the percent composition for each element by dividing the total mass of that element by the molecular mass of the compound and then multiply by 100 to convert it to a percentage.

For example, if you have a compound with a molecular formula of H₂O, you'd find the mass of hydrogen (H) to be about 1 g/mol (since there are 2 atoms, it's 2 g/mol for hydrogen), and the mass of oxygen (O) to be about 16 g/mol. The molecular mass of water is 18 g/mol (2 g/mol for hydrogen + 16 g/mol for oxygen). The percent composition of hydrogen is:

$\frac{2}{18}\times 100=11.11\%$

and for oxygen, it's:

$\frac{16}{18}\times 100=88.89\%$

How do you find mass percent from a chemical formula?

To find the mass percent of an element in a compound from its chemical formula, follow these steps:

1. Determine the molar mass of the compound by adding the atomic masses of all atoms in the formula. You can find atomic masses on the periodic table.
2. Find the total mass of the element in question within one mole of the compound. Do this by multiplying the atomic mass of the element by the number of atoms of that element in the compound.
3. Calculate the mass percent by dividing the mass of the element in the compound (from step 2) by the molar mass of the compound (from step 1) and then multiply by 100 to get a percentage.

For example, in water (H₂O), the molar mass is 18.02 g/mol (16.00 g/mol from oxygen + 2 x 1.01 g/mol from hydrogen). The mass of hydrogen is 2 x 1.01 g/mol. The mass percent of hydrogen is:

$\frac{2.02g/mol}{18.02g/mol}\times 100=11.21\%$

Repeat this process for each element in the compound to find their respective mass percents.