Solutions: Mass Percent: Videos & Practice Problems

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 both the solute (NaOH) and the solvent. In this case, the solvent's mass can be determined by subtracting the mass of the solute from the total mass of the solution:

Mass of Solvent = Total Mass of Solution - Mass of Solute

Using the previous example, the mass of the solvent would be:

Mass of Solvent = 100 g - 23 g = 77 g

This breakdown illustrates that a solution consists of both solute and solvent, and understanding this relationship is essential for solving problems related to mass percent. As you practice calculating mass percent, remember to consider both components of the solution to enhance your comprehension and problem-solving skills.

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:

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

In this scenario, the solute is urea, with a mass of 12 grams. We denote the mass of water to be added as \(x\) grams. The total mass of the solution will then be \(12 + x\) grams. Plugging these values into the mass percent formula, we have:

18.3 = \(\frac{12}{12 + x} \times 100\)

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

18.3(12 + x) = 1200

Distributing the 18.3 gives:

219.6 + 18.3x = 1200

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

18.3x = 980.4

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

\(x = \frac{980.4}{18.3} \approx 53.6\) grams

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

\(x \approx 0.0536\) kg

In summary, to prepare an 18.3% by mass solution of urea, you need to add approximately 0.0536 kilograms of water. This example illustrates the importance of using the mass percent formula to isolate the variable of interest and converting units as necessary for the final answer.

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 determining the total mass of the solution, which is the sum of the mass of KBr and the mass of water. Here, you have:

• Mass of KBr = 51 grams
• Volume of water = 310 mL

To convert the volume of water to mass, you can use the density of water, which is approximately 1 gram per milliliter. Therefore, the mass of water is:

Mass of water = 310 mL × 1 g/mL = 310 grams

Now, you can calculate the total mass of the solution:

Total mass of solution = Mass of KBr + Mass of water = 51 g + 310 g = 361 grams

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%. Remember, the key steps involve knowing the density of water to convert volume to mass and then applying the mass percent formula correctly.