Mass Percent: Videos & Practice Problems

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 Calculation

Mass percent, often referred to as weight percent, quantifies the percentage of a specific element within a compound. This concept is closely tied to the molar mass, which is the mass in grams of the selected element relative to one mole of the compound. The formula for calculating mass percent can be expressed as:

\[\text{Mass Percent} = \left( \frac{\text{Mass of Element}}{\text{Mass of Compound}} \right) \times 100\]

By applying this formula, one can effectively determine the percent composition of any particular element in a compound. Understanding mass percent is crucial for various applications in chemistry, including stoichiometry, where it aids in the analysis of chemical reactions and the formulation of compounds.

Mass Percent Example

To determine the weight percent of carbon in sucrose, we start by recognizing that weight percent and mass percent are equivalent. The formula for calculating mass percent is:

Mass Percent of Carbon = \(\frac{\text{grams of carbon}}{\text{grams of sucrose}} \times 100\)

Sucrose has the chemical formula \(C_{12}H_{22}O_{11}\), which indicates it contains 12 carbon (C) atoms, 22 hydrogen (H) atoms, and 11 oxygen (O) atoms. To find the total mass of each element, we multiply the number of atoms by their respective atomic masses from the periodic table:

• Mass of Carbon: \(12 \times 12.01 \, \text{g/mol} = 144.12 \, \text{g}\)
• Mass of Hydrogen: \(22 \times 1.008 \, \text{g/mol} = 22.176 \, \text{g}\)
• Mass of Oxygen: \(11 \times 16.00 \, \text{g/mol} = 176.00 \, \text{g}\)

Next, we sum these masses to find the total mass of sucrose:

Total Mass of Sucrose = \(144.12 \, \text{g} + 22.176 \, \text{g} + 176.00 \, \text{g} = 342.296 \, \text{g}\)

Now, we can substitute the values into the mass percent formula:

Mass Percent of Carbon = \(\frac{144.12 \, \text{g}}{342.296 \, \text{g}} \times 100 \approx 42.1\%\)

Thus, the weight percent of carbon in sucrose is approximately 42.1%.

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

Answer: (C)

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.