Solutions: Mass Percent: Videos & Practice Problems

Mass percent is the ratio of the mass of a solute to the total mass of a solution, expressed as a percentage. For example, a 23% NaOH solution contains 23 grams of NaOH in 100 grams of solution, with 77 grams being the solvent. Understanding this concept is crucial for calculations involving solutions, as it allows for the breakdown of components into solute and solvent, facilitating further analysis in chemistry, including stoichiometry and concentration calculations.

Solutions: Mass Percent Concept 1

Mass or weight percent is a crucial concept in chemistry that quantifies the concentration of a specific element or compound within a solution. It is calculated using the formula:

Mass Percent = \(\frac{\text{Mass of Solute}}{\text{Total Mass of Solution}} \times 100\)

For instance, if a solution contains 23% sodium hydroxide (NaOH), this indicates that there are 23 grams of NaOH in a total of 100 grams of the solution. This percentage implies that the solution is composed of 23 grams of solute (NaOH) and the remaining mass, which is 77 grams, is the solvent. Thus, the total mass of the solution can be expressed as:

Total Mass of Solution = Mass of Solute + Mass of Solvent

In this example, the breakdown is as follows:

Total Mass of Solution = 23 g (NaOH) + 77 g (solvent) = 100 g

Understanding how to decompose mass percent into its solute and solvent components is essential for solving related problems. This foundational knowledge will aid in tackling more complex questions regarding mass percent in future exercises.

Solutions: Mass Percent Example 1

To determine the amount of water in kilograms needed to prepare an 18.3% by mass solution of urea, we start by understanding the concept of mass percent. The mass percent formula is given by:

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

In this scenario, the mass of the solute (urea) is 12 grams, and we need to find the mass of water (denoted as \(x\)) that will be added to achieve the desired concentration. The total mass of the solution is the sum of the mass of urea and the mass of water, which can be expressed as \(12 + x\) grams.

Substituting the known values into the mass percent formula, we have:

\[18.3 = \left( \frac{12}{12 + x} \right) \times 100\]

To eliminate the fraction, we multiply both sides by \(12 + x\):

\[18.3(12 + x) = 1200\]

Distributing \(18.3\) gives:

\[219.6 + 18.3x = 1200\]

Next, we isolate \(x\) by subtracting \(219.6\) from both sides:

\[18.3x = 980.4\]

Now, divide both sides by \(18.3\) to solve for \(x\):

\[x = \frac{980.4}{18.3} \approx 53.6 \text{ grams of } H_2O\]

Since the final answer is required in kilograms, we convert grams to kilograms using the conversion factor \(1 \text{ kg} = 1000 \text{ g}\):

\[x \approx 0.0536 \text{ kg of } H_2O\]

In summary, when calculating mass percent solutions, it is crucial to isolate the variable representing the mass of the solvent, and then convert the final result to the desired units. This method can be applied to similar problems involving mass percent solutions.

Solutions: Mass Percent Example 2

To calculate the mass percent of potassium bromide (KBr) in a solution, you can use the formula for mass percent, which is:

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

In this case, the solute is KBr, and the solvent is water. You start by identifying the mass of KBr, which is given as 51 grams. Next, you need to determine the total mass of the solution, which consists of both the solute and the solvent.

Since the solution is prepared by dissolving KBr in water, you need to calculate the mass of the water. The volume of water used is 310 mL. Knowing that the density of water is approximately 1 gram per milliliter, you can convert the volume of water to mass:

Mass of water = Volume of water × Density of water = \(310 \, \text{mL} \times 1 \, \text{g/mL} = 310 \, \text{g}\)

Now, you can find the total mass of the solution by adding the mass of KBr and the mass of water:

Total mass of solution = Mass of KBr + Mass of water = \(51 \, \text{g} + 310 \, \text{g} = 361 \, \text{g}\)

Now, substitute the values into the mass percent formula:

Mass Percent = \(\frac{51 \, \text{g}}{361 \, \text{g}} \times 100 \approx 14.1\%\)

This calculation shows that the mass percent of potassium bromide in the solution is approximately 14.1%. Remembering the density of water as 1 g/mL is crucial for converting volume to mass, which is a key step in solving this type of problem.

Here’s what students ask on this topic:

What is mass percent in chemistry?

Mass percent, also known as weight percent, is a way of expressing the concentration of a component in a mixture or solution. It is calculated as the mass of the solute divided by the total mass of the solution, multiplied by 100. The formula is:

$\frac{\mathrm{mass} \mathrm{of} \mathrm{solute}}{\mathrm{total} \mathrm{mass} \mathrm{of} \mathrm{solution}} \times 100$

For example, a 23% NaOH solution means there are 23 grams of NaOH in 100 grams of the solution.

How do you calculate mass percent of a solution?

To calculate the mass percent of a solution, use the formula:

$\frac{\mathrm{mass} \mathrm{of} \mathrm{solute}}{\mathrm{total} \mathrm{mass} \mathrm{of} \mathrm{solution}} \times 100$

For example, if you have 10 grams of NaCl dissolved in 90 grams of water, the total mass of the solution is 100 grams. The mass percent is:

$\frac{10}{100} \times 100 = 10 \%$

What is the difference between mass percent and volume percent?

Mass percent and volume percent are both ways to express concentration, but they differ in what they measure. Mass percent is the ratio of the mass of the solute to the total mass of the solution, multiplied by 100. Volume percent, on the other hand, is the ratio of the volume of the solute to the total volume of the solution, multiplied by 100. Mass percent is useful for solid and liquid solutes, while volume percent is often used for liquid-liquid solutions.

How do you convert mass percent to molarity?

To convert mass percent to molarity, follow these steps:

1. Calculate the mass of the solute in grams.
2. Convert the mass of the solute to moles using its molar mass.
3. Determine the volume of the solution in liters.
4. Use the formula:

$\frac{\mathrm{moles} \mathrm{of} \mathrm{solute}}{\mathrm{volume} \mathrm{of} \mathrm{solution} \mathrm{(L)}}$

For example, for a 10% NaCl solution with a density of 1.1 g/mL, calculate the moles of NaCl and divide by the volume in liters to find the molarity.

Why is mass percent important in chemistry?

Mass percent is important in chemistry because it provides a straightforward way to express the concentration of a solute in a solution. It is particularly useful in stoichiometry, where precise measurements of reactants and products are crucial. Mass percent allows chemists to easily calculate the amounts of substances needed for reactions, analyze the composition of mixtures, and prepare solutions with specific concentrations. It also helps in quality control and standardization in industrial processes.