Mass Percent

The mass percent method is a way to describe the concentration of a solution by indicating the percentage of solute in the total mass of the solution. It reflects the composition and shows how much solute is present in a given volume. Solute solubility can be expressed in terms of mass or moles.

1.0Introduction 

Mass percent is a method used to express the concentration of a solution. It indicates the proportion of a component in a mixture or solution. Specifically, mass percent defines the amount of solute and the total mass of the solution.

The mass percentage helps to determine the composition of a solution, showing how much solute is present in a specific volume. Solute solubility can be quantified by either mass or moles.

Understanding mass percent can help you gain insights into the solution's composition. It reveals the exact quantity of solute present in a given volume. This measurement can be expressed in mass (grams) or moles, depending on the context.

2.0Mass Percent Formula

$MassPercent=(\frac{MassofSolute}{MassofSolution})\times 100$

Where:

• Mass of Solute = Mass of the substance being dissolved.
• Mass of Solution = Mass of the solute + Mass of the solvent.
• Mass Percent Range: Mass percent can range from 0% to 100%. A mass percent of 0% means no solute is present, while 100% means the solution consists entirely of the solute.
• Solubility and Mass Percent: Mass percent helps understand a substance's solubility. Higher mass percent values indicate a more concentrated solution.
• Use in Problem Solving: Mass percent is often used in stoichiometric calculations to determine substance purity and yield in chemical reactions.

Note

• Unit: Mass percent has no units; it is expressed as a percentage.
• Range: Mass percent can be between 0% (pure solvent) and 100% (pure solute).
• Application: It is used to determine concentrations in laboratory solutions and is often required for stoichiometric calculations in chemical reactions.

3.0Solved Examples

Example 1: A solution dissolves 10 grams of sodium chloride (NaCl) in 90 grams of water. Calculate the mass percent of NaCl in the solution.

Solution:

1. Mass of Solute (NaCl) = 10 grams
2. Mass of Solvent (Water) = 90 grams
3. Mass of Solution = Mass of Solute + Mass of Solvent = 10 g + 90 g = 100 grams

Using the mass percent formula:

$MassPercent=(\frac{10grams}{100grams})\times 100=10$

Answer: The mass percent of NaCl is 10%.

Example 2: If a solution has a mass percent of 15% sugar and the total mass of the solution is 200 grams, what is the mass of the sugar present?

Solution:

1. Mass Percent of Sugar = 15%
2. Total Mass of Solution = 200 grams

Using the formula:

$MassofSolute=(\frac{MassPercent}{100})\times MassofSolution$

$MassofSolute=(\frac{15}{100})\times 200=30grams$

Answer: The mass of sugar in the solution is 30 grams.

Example 3: How much water must be added to 50 grams of a 20% NaCl solution to make it a 10% NaCl?

Solution:

1. Initial Mass Percent = 20%
2. Initial Mass of Solution = 50 grams
3. Mass of NaCl in the original solution = 20% of 50 grams = 20/100 X 50 = 10 grams

To dilute to 10% NaCl:

$FinalMassPercent=(\frac{MassofNaCl}{FinalMassofSolution})\times 100$

Let the final mass of the solution be x grams:

10 = (10/x) X 100

Or x = 100 grams

So, the final mass of the solution should be 100 grams.
Water to be added = 100 grams - 50 grams = 50 grams.

Answer: 50 grams of water must be added.