Percent Concentration Practice Questions with Answers

Concept Explanation

Percent concentration is a measure of the amount of solute present in a given amount of solution, expressed as a percentage of the total mass or volume. This fundamental concept in chemistry allows scientists and students to quantify the strength of a mixture. Generally, it is calculated using one of three primary methods: mass percent (m/m), volume percent (v/v), or mass-volume percent (m/v). Understanding these calculations is essential before moving on to more complex topics like molarity formula calculations or stoichiometry.

The general formula for percent concentration is:

Percent Concentration = (Amount of Solute / Amount of Solution) × 100

It is critical to remember that the "Amount of Solution" is the sum of the solute and the solvent. For example, if you dissolve 5 grams of salt in 95 grams of water, the total solution mass is 100 grams. According to the IUPAC definitions, concentration can be expressed in various units, but percent composition remains one of the most common in household products and clinical settings.

Types of Percent Concentration

• Mass Percent (% m/m): Used when both solute and solvent are measured by mass (usually grams).

• Volume Percent (% v/v): Common for liquid-liquid mixtures, such as rubbing alcohol or beverages.

• Mass-Volume Percent (% m/v): Frequently used in medical contexts (e.g., saline IV bags), measuring grams of solute per 100 mL of solution.

Solved Examples

Reviewing these worked examples will help you master the basic calculations before attempting the practice set.

1. Calculating Mass Percent: A solution is prepared by dissolving 15.0 g of sodium chloride (NaCl) in 235.0 g of water. What is the mass percent of the solution?

   1. Identify the mass of the solute: 15.0 g.

   2. Identify the mass of the solvent: 235.0 g.

   3. Calculate the total mass of the solution: 15.0 g + 235.0 g = 250.0 g.

   4. Apply the formula: (15.0 / 250.0) × 100 = 6.0%.

   5. The concentration is 6.0% (m/m).

2. Calculating Volume Percent: If 40 mL of pure ethanol is diluted with water to a final volume of 200 mL, what is the volume percent concentration?

   1. Identify the volume of the solute: 40 mL.

   2. Identify the total volume of the solution: 200 mL (Note: the problem gives the final volume, so no addition is needed).

   3. Apply the formula: (40 / 200) × 100 = 20%.

   4. The concentration is 20% (v/v).

3. Calculating Mass of Solute: How many grams of sugar are needed to make 500 g of a 5% (m/m) sugar solution?

   1. Identify the total mass of the solution: 500 g.

   2. Identify the target percentage: 5%.

   3. Rearrange the formula: Mass of solute = (Percent / 100) × Total mass.

   4. Calculate: (5 / 100) × 500 = 25 g.

   5. You need 25 grams of sugar.

Practice Questions

Test your knowledge with these problems. Ensure you differentiate between the mass of the solvent and the total mass of the solution.

1. A chemist dissolves 12 grams of glucose in 108 grams of water. What is the mass percent (% m/m) of glucose in this solution?

2. A 250 mL bottle of mouthwash contains 45 mL of ethanol. Calculate the volume percent (% v/v) of ethanol.

3. How many grams of potassium chloride (KCl) are required to prepare 400 grams of a 15% (m/m) solution?

4. A saline solution is labeled as 0.9% (m/v) NaCl. How many grams of NaCl are present in 500 mL of this solution?

5. If you mix 50 mL of vinegar with 150 mL of water, what is the volume percent of vinegar in the resulting solution?

6. You have 30 grams of a 20% (m/m) sugar solution. How much sugar is in the solution?

7. A student needs to make 1.5 kg of a 10% (m/m) solution of NaOH. What mass of NaOH and what mass of water should they use?

8. 75 mL of propanol is mixed with enough water to make 250 mL of solution. What is the % v/v concentration?

9. A 300 g solution of calcium chloride contains 45 g of the solute. What is the mass percent of the solvent?

10. An alloy is made by melting 4.5 kg of copper with 0.5 kg of tin. What is the percent concentration of tin in the alloy?

Answers & Explanations

1. 10% (m/m). Total mass = 12g + 108g = 120g. (12 / 120) × 100 = 10%.

2. 18% (v/v). (45 mL / 250 mL) × 100 = 18%.

3. 60 grams. (15 / 100) × 400g = 60g.

4. 4.5 grams. 0.9% means 0.9g per 100 mL. So, (0.9 / 100) × 500 mL = 4.5g.

5. 25% (v/v). Total volume = 50 mL + 150 mL = 200 mL. (50 / 200) × 100 = 25%.

6. 6 grams. (20 / 100) × 30g = 6g.

7. 150g NaOH and 1350g water. NaOH = (10 / 100) × 1500g = 150g. Water = 1500g - 150g = 1350g.

8. 30% (v/v). (75 / 250) × 100 = 30%.

9. 85%. Mass percent solute = (45 / 300) × 100 = 15%. Solvent percent = 100% - 15% = 85%.

10. 10%. Total mass = 4.5kg + 0.5kg = 5.0kg. (0.5 / 5.0) × 100 = 10%.

Quick Quiz

Frequently Asked Questions

What is the difference between mass percent and molarity?

Mass percent measures the ratio of the solute's mass to the solution's total mass, whereas molarity measures moles of solute per liter of solution. You can learn more about these differences in our molarity vs molality guide.

Does temperature affect percent concentration?

Mass percent (% m/m) does not change with temperature because mass is independent of temperature. However, volume-based concentrations like % v/v or % m/v can change slightly as liquids expand or contract with temperature fluctuations.

Is the solvent always water in these problems?

While water is the most common "universal solvent" in chemistry, any substance can act as a solvent. In alloys, the solvent is the metal present in the largest amount, and in air, nitrogen acts as the solvent for other gases.

How do I convert percent concentration to grams?

To find the mass of solute, multiply the total mass of the solution by the percent concentration (expressed as a decimal). This is a common step in converting grams to moles for stoichiometry problems.

What is a common real-world example of mass-volume percent?

Medical IV fluids, such as 0.9% Normal Saline or 5% Dextrose, are classic examples of mass-volume percent used in healthcare. These are prepared according to strict FDA standards to ensure patient safety.

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