Solutions: Mass Percent: Videos & Practice Problems

Understanding mass percent is crucial when working with solutions in chemistry. Mass percent is calculated by taking the grams of solute, dividing it by the grams of the total solution, and then multiplying by 100. This formula allows you to express the concentration of a component in a solution. For example, a 2.50% sodium hydroxide solution means there are 2.50 grams of sodium hydroxide in every 100 grams of the solution. This percentage can also serve as a conversion factor, aiding in the transition between different units of concentration, such as molarity. Remember, the mass percent provides a straightforward method to quantify the ratio of solute to solution, which is essential for precise chemical formulation and reactions.

Solutions: Mass Percent

In the study of solutions, understanding mass percent is crucial. Mass percent is defined as the ratio of the mass of the solute to the total mass of the solution, expressed as a percentage. The formula for calculating mass percent is:

\[\text{Mass Percent} = \left( \frac{\text{grams of solute}}{\text{grams of solution}} \right) \times 100\%

This means that to find the mass percent, you divide the mass of the solute by the mass of the entire solution and then multiply the result by 100. This concept is essential when solving problems related to solutions, as it allows for a clear understanding of the concentration of solute within a given solution. Always keep this formula in mind when approaching questions involving mass percent.

Mass Percent Calculation Example

To calculate the mass percent of a solution, you need to determine the mass of the solute (the substance being dissolved) and the total mass of the solution (which includes both the solute and the solvent). In this case, potassium chloride (KCl) is the solute, and water is the solvent.First, identify the mass of the solute:- Mass of KCl = 34.1 gramsNext, calculate the mass of the solvent:- Mass of water = 150 gramsNow, find the total mass of the solution by adding the mass of the solute and the mass of the solvent:\[\text{Total mass of solution} = \text{Mass of KCl} + \text{Mass of water} = 34.1 \, \text{g} + 150 \, \text{g} = 184.1 \, \text{g}\]The mass percent of the solution can be calculated using the formula:\[\text{Mass percent} = \left( \frac{\text{Mass of solute}}{\text{Total mass of solution}} \right) \times 100\]Substituting the values:\[\text{Mass percent} = \left( \frac{34.1 \, \text{g}}{184.1 \, \text{g}} \right) \times 100 \approx 18.5\%\]Thus, the mass percent of the potassium chloride solution is approximately 18.5%. This value indicates the proportion of the solute in the total solution, which is essential for understanding the concentration of the solution in various applications.

Calculate the amount of water (in kilograms) that must be added to 12.0 g of urea, (NH₂)₂CO, in the preparation of a 18.3 percent by mass solution. The molar mass of urea, (NH₂)₂CO, is 60.055 g/mol.

• A. 0.644 kg
• B. 0.0535 kg
• C. 6.44 kg
• D. 0.0755 kg

Answer: (B)

Mass Percent Conversion

When working with solutions, understanding the concept of mass percent is crucial for conversions to other unit terms, such as molarity. For instance, a 2.50% sodium hydroxide (NaOH) solution indicates that there are 2.50 grams of NaOH in every 100 grams of the solution. This relationship can be expressed as:

\( \frac{2.50 \, \text{g NaOH}}{100 \, \text{g solution}} \)

This ratio serves as a conversion factor, allowing for flexibility in calculations. The reciprocal can also be utilized, which would be:

\( \frac{100 \, \text{g solution}}{2.50 \, \text{g NaOH}} \)

By recognizing that the mass percent value represents a ratio of the solute to the total solution mass, one can effectively expand and manipulate this information for various calculations. This foundational understanding is essential for accurately determining concentrations and performing further chemical analyses.

Molarity Calculation Example

To determine the molarity of a sulfuric acid solution that is 5% sulfuric acid with a density of 0.9918 grams per milliliter, we start by understanding the concept of molarity, which is defined as the number of moles of solute per liter of solution.

Given that the solution is 5% sulfuric acid, this means there are 5 grams of sulfuric acid in every 100 grams of solution. To convert grams of sulfuric acid to moles, we use the molar mass of sulfuric acid (H₂SO₄), which is approximately 98.086 grams per mole. Thus, we can calculate the moles of sulfuric acid:

\[\text{Moles of } H_2SO_4 = \frac{5 \text{ grams}}{98.086 \text{ grams/mole}} \approx 0.0510 \text{ moles}\]

Next, we need to convert the mass of the solution to volume using the density. The density of the solution is 0.9918 grams per milliliter, which allows us to find the volume of 100 grams of solution:

\[\text{Volume} = \frac{100 \text{ grams}}{0.9918 \text{ grams/mL}} \approx 100.83 \text{ mL}\]

To convert milliliters to liters, we use the conversion factor that 1 mL is equal to \(10^{-3}\) L:

\[\text{Volume in liters} = 100.83 \text{ mL} \times 10^{-3} \text{ L/mL} \approx 0.10083 \text{ L}\]

Now, we can calculate the molarity (M) of the sulfuric acid solution by dividing the moles of sulfuric acid by the volume of the solution in liters:

\[\text{Molarity} = \frac{0.0510 \text{ moles}}{0.10083 \text{ L}} \approx 0.5056 \text{ M}\]

Finally, rounding to two significant figures, the molarity of the sulfuric acid solution is approximately 0.51 M. This calculation illustrates the process of converting mass percent to molarity using molar mass and density, essential for understanding solution concentrations in chemistry.

A solution was prepared by dissolving 51.0 g of KBr in 310 mL of water. Calculate the mass percent of KBr in the solution.

• A. 16.4%
• B. 14.1%
• C. 19.7%
• D. 0.141%

Answer: (B)

An aqueous LiNO₂ solution is made using 90.3 g LiNO₂ and diluting it to a total volume of 1.72 L. If the density of the solution is 1.20 g/mL, what is the mass percent of the solution?

• A. 4.37%
• B. 5.25%
• C. 4.19%
• D. 4.99%

Answer: (A)

Determine the percent sulfuric acid by mass of a 1.37 m aqueous solution of H₂SO₄.

• A. 0.1342%
• B. 11.85%
• C. 13.42%
• D. 13.98%

Answer: (B)

Here’s what students ask on this topic:

What is the formula for calculating mass percent in a solution?

The formula for calculating mass percent in a solution is:

$\frac{\mathrm{grams} \mathrm{of} \mathrm{solute}}{\mathrm{grams} \mathrm{of} \mathrm{solution}} \times 100$

This means you take the mass of the solute, divide it by the total mass of the solution, and then multiply by 100 to get the percentage. This formula helps express the concentration of a component in a solution.

How do you convert mass percent to molarity?

To convert mass percent to molarity, follow these steps:

1. Use the mass percent to find the mass of solute in 100 grams of solution.

2. Convert the mass of solute to moles using its molar mass.

3. Determine the volume of the solution in liters (assuming the density is known or approximated).

4. Use the formula for molarity:

$\frac{\mathrm{moles} \mathrm{of} \mathrm{solute}}{\mathrm{liters} \mathrm{of} \mathrm{solution}}$

This will give you the molarity of the solution.

What does a 2.50% sodium hydroxide solution mean?

A 2.50% sodium hydroxide (NaOH) solution means that there are 2.50 grams of sodium hydroxide dissolved in every 100 grams of the solution. This percentage indicates the mass of the solute (NaOH) relative to the total mass of the solution. It can also be used as a conversion factor, where 2.50 grams of NaOH is equivalent to 100 grams of the solution, and vice versa.

Why is understanding mass percent important in chemistry?

Understanding mass percent is crucial in chemistry because it provides a straightforward method to quantify the concentration of a solute in a solution. This is essential for precise chemical formulation and reactions. Mass percent helps in preparing solutions with accurate concentrations, converting between different units of concentration, and ensuring consistency in experimental results. It also aids in understanding the composition of mixtures and solutions, which is fundamental in various chemical processes and applications.

How can mass percent be used as a conversion factor?

Mass percent can be used as a conversion factor by expressing the percentage as a ratio of the mass of solute to the mass of the solution. For example, a 2.50% sodium hydroxide solution can be written as 2.50 grams of NaOH per 100 grams of solution. This ratio can be used to convert between the mass of the solute and the mass of the solution. It can also be inverted to have 100 grams of solution per 2.50 grams of NaOH, depending on the conversion needed. This flexibility makes mass percent a useful tool in various calculations involving solutions.