Solutions: Mass Percent in Chemistry

Solutions

Mass Percent

Mass percent (also called weight percent) is a way to express the concentration of an element or compound in a solution. It is calculated as the ratio of the mass of the solute to the total mass of the solution, multiplied by 100%.

• Mass Percent Formula:

$\text{Mass Percent} = \frac{\text{Mass Component}}{\text{Total Mass}} \times 100\%$

• Solute: The substance dissolved in the solution (e.g., NaOH, KBr).

• Solution: The total mixture of solute and solvent.

Example Calculation

Suppose we have a solution that is 23.0% NaOH by mass. This means:

$\frac{23.0\ \text{grams NaOH}}{100\ \text{grams Solution}}$

• 23.0 grams of NaOH are present in every 100 grams of solution.

Worked Example 1

Problem: Calculate the amount of water (in kilograms) that must be added to 12.0 g of urea (NH2CONH2) to prepare a 14.3% by mass solution.

• Step 1: Let the mass of water to be added be x grams.

• Step 2: Total mass of solution = mass of urea + mass of water = 12.0 g + x

• Step 3: Set up the equation using the mass percent formula:

$\frac{12.0}{12.0 + x} \times 100 = 14.3$

• Step 4: Solve for x:

$12.0 \times 100 = 14.3(12.0 + x)$ $1200 = 171.6 + 14.3x$ $1200 - 171.6 = 14.3x$ $1028.4 = 14.3x$ $x = \frac{1028.4}{14.3} = 71.9$ grams

• Step 5: Convert to kilograms if needed: $71.9\ \text{g} = 0.0719\ \text{kg}$

Worked Example 2

Problem: A solution was prepared by dissolving 5.0 g of KBr in 210 g of water. Calculate the mass percent of KBr in the solution.

• Step 1: Total mass of solution = 5.0 g + 210 g = 215 g

• Step 2: Use the mass percent formula:

$\text{Mass percent} = \frac{5.0}{215} \times 100 = 2.33\%$

• Conclusion: The solution contains 2.33% KBr by mass.

Key Points

• Mass percent is a common way to express solution concentration in chemistry.

• It is useful for preparing solutions with precise concentrations for laboratory and industrial applications.

• Always ensure units are consistent when performing calculations.