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

Mass percent, or weight percent, quantifies the percentage of a specific element in a compound. It is calculated by dividing the mass of the element by the total mass of the compound and multiplying by 100. This concept is essential for determining the percent composition of elements, aiding in stoichiometry and chemical analysis. Understanding mass percent is crucial for students studying chemical reactions, as it relates to concepts like molar mass and empirical formulas, enhancing their grasp of chemical properties and reactions.

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 in grams of the selected element and 1 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 used, then multiplying that by 100. Using this helps us to determine the percent composition of any particular element within a given compound.

Mass Percent Example

Here it says to calculate the mass percent of carbon within sodium carbonate. So sodium carbonate is Na₂CO₃. We're looking for the mass percent of carbon, so that means we're gonna need the grams of carbon over the grams of the compound itself times 100. Alright. So sodium carbonate has within it 2 sodiums, 1 carbon, and 3 oxygens. We're gonna multiply them each by their atomic masses from the periodic table. So sodium is 22.99 grams, carbon is 12.01 grams and oxygen is 16 grams. So multiplying them out we get 45.98 grams, 12.01 grams, and 48 grams. Add them all together, that comes out to 105.99 grams. Take those numbers. So for carbon, its mass is 12.01, and for the compound itself it's 105.99. So we do:

12.01 105.99 × 100

When we do that, we get 11.3% as the mass percent of carbon within sodium carbonate.

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

Here it says, determine the percent composition of nitrogen and oxygen within nitrogen dioxide. Alright. So what we need to do here is we need to find the mass percent of nitrogen, and then the mass percent of oxygen. So remember here this will be the grams of nitrogen within the grams of the compound times 100. And for oxygen, it'll be grams of oxygen divided by the grams of the compound also times 100. If we look at nitrogen dioxide, it's composed of 1 nitrogen, and it's composed of 2 oxygens. Multiply them by their atomic masses, which you find from the periodic table. Nitrogen weighs 14.01 grams, oxygen weighs 16 grams. Multiply them, and then add up your totals, so the mass of the compound is 46.01 grams. So that's the number that goes here and here. And then all we need to do is plug in what we have for nitrogen which is then when we do that we're going to get here for nitrogen, its mass percent as 30.4%. And then for oxygen, you could plug it into this formula. You could also say that we know that 100% of the compound is made up of just nitrogen and oxygen. So you could say 100% minus 30.4% gives us also the percentage of oxygen. So this will come out to 69.6%. So here we have both the mass percent of nitrogen and oxygen. Giving the percentages of all the elements within the compound are classified as the percent composition of the compound. So the compound is made up of 30.4% nitrogen and 69.6% oxygen.

Here’s what students ask on this topic:

What is mass percent and how is it calculated?

Mass percent, also known as weight percent, quantifies the percentage of a specific element in a compound. It is calculated using the formula:

$\frac{\mathrm{mass} \mathrm{of} \mathrm{element}}{\mathrm{mass} \mathrm{of} \mathrm{compound}} \times 100$

This formula helps determine the percent composition of any particular element within a given compound. For example, if you have a compound with a total mass of 100 grams and it contains 20 grams of a specific element, the mass percent of that element would be:

$\frac{20}{100} \times 100 = 20 \%$

Why is mass percent important in chemistry?

Mass percent is crucial in chemistry because it helps determine the percent composition of elements in compounds. This information is essential for stoichiometry, which involves calculating the quantities of reactants and products in chemical reactions. Understanding mass percent also aids in empirical formula determination, which is the simplest ratio of elements in a compound. Additionally, it helps in chemical analysis and quality control in various industries, ensuring that compounds meet specific standards. Overall, mass percent enhances the understanding of chemical properties and reactions, making it a fundamental concept for students and professionals in the field.

How do you find the mass percent of an element in a compound?

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

1. Determine the molar mass of the compound by adding the atomic masses of all the elements in the compound.
2. Find the total mass of the element of interest in one mole of the compound.
3. Use the mass percent formula:

$\frac{\mathrm{mass} \mathrm{of} \mathrm{element}}{\mathrm{mass} \mathrm{of} \mathrm{compound}} \times 100$

For example, to find the mass percent of hydrogen in water (H₂O):

1. Molar mass of H₂O = 2(1.01) + 16.00 = 18.02 g/mol
2. Mass of hydrogen in one mole of H₂O = 2(1.01) = 2.02 g
3. Mass percent of hydrogen = (2.02 / 18.02) × 100 ≈ 11.21%

What is the difference between mass percent and mole percent?

Mass percent and mole percent are both ways to express the composition of a mixture or compound, but they differ in what they measure:

• Mass Percent: This measures the mass of a component divided by the total mass of the mixture or compound, multiplied by 100. It is useful for understanding the weight contribution of each element in a compound.
• Mole Percent: This measures the number of moles of a component divided by the total number of moles in the mixture, multiplied by 100. It is useful for understanding the proportion of each component in terms of the number of particles.

For example, in a solution of salt in water, mass percent would tell you the weight percentage of salt, while mole percent would tell you the percentage of salt molecules relative to the total number of molecules in the solution.

How is mass percent used in stoichiometry?

Mass percent is used in stoichiometry to determine the amount of each element in a compound, which is essential for calculating reactants and products in chemical reactions. By knowing the mass percent of elements, you can convert between mass and moles, allowing you to use balanced chemical equations to predict the quantities of substances involved. For example, if you know the mass percent of hydrogen in water, you can calculate how much hydrogen gas will be produced from a given amount of water through electrolysis. This makes mass percent a valuable tool for solving stoichiometric problems and understanding chemical reactions.