Understanding 10% W/V Concentration: Is It 10g of Solute in 90mL of Solvent?

Understanding 10% W/V Concentration: Is It 10g Solute in 90mL Solvent?

10% W/V concentration means 10 grams of solute dissolved to make a total volume of 100 milliliters of solution, not 10g solute in 90 mL of solvent. This distinction is important for accurate solution preparation in chemistry and biology. The weight/volume percentage (w/v%) refers to the mass of solute per total final volume of solution, not just the volume of solvent.

What Does 10% W/V Mean?

The abbreviation “% W/V” stands for weight per volume percentage. It is a measure used to express the concentration of a solution. The official and most accepted definition, especially in laboratory practice, is:

• 10% W/V means 10 grams of solute per 100 milliliters of total solution volume.
• The total volume refers to the volume after the solute has been dissolved and all components combined.
• For example, a common method involves placing 10 g of solute into a 100 mL volumetric flask, adding solvent until the total volume reaches exactly 100 mL.

This means you do not simply add solute to 90 mL of solvent, expecting a 10% solution. The final volume, including both solute and solvent, must reach 100 mL.

Why Not 10g Solute in 90 mL Solvent?

The belief that 10% W/V equals 10 g solute in 90 mL solvent comes from a misunderstanding about how volume and concentration relate. This view assumes solute volume is negligible or that the solvent volume is adjusted accordingly, which is incorrect.

• The volume of a solution is not just the volume of the solvent because solute particles occupy space.
• When you dissolve 10 g of a substance, it increases the total volume beyond 90 mL.
• A 10% W/V solution requires the final volume after solute addition to be 100 mL, not just solvent volume.

Mathematical Clarification

The w/v percentage is calculated as:

% W/V = (mass of solute in grams / volume of solution in milliliters) × 100%

If the volume used is 90 mL solvent, and the intention is a 10% W/V solution, then the mass of solute needed satisfies:

(mass solute / 90 mL) × 100 = 10% → mass solute = 9 g

This means adding 9 grams of solute to 90 mL solvent yields a 10% W/V based on solvent volume alone, but this is not the standard interpretation.

In actual practice, you must ensure the total volume containing solute and solvent is 100 mL. Starting with 90 mL solvent and adding 10 g solute makes the volume slightly more than 90 mL but less than 100 mL, meaning concentration is not exactly 10% W/V.

Practical Preparation of 10% W/V Solution

Standard lab practice to prepare a 10% W/V solution includes:

1. Weigh 10 g of the solute accurately.
2. Add the solute into a volumetric flask of 100 mL capacity.
3. Add solvent gradually to dissolve the solute completely.
4. After the solute dissolves, fill the flask to the 100 mL mark using solvent.
5. Mix thoroughly to ensure homogeneity.

This method guarantees the final volume is 100 mL, matching the 10% W/V definition.

Common Misconceptions About W/V Concentration

• Confusing solvent volume with total solution volume leads to errors.
• Interpreting “10% W/V” as 10 g solute + 90 mL solvent is a common mistake.
• Ignoring the volume contribution of solute can cause inaccurate concentration.
• Some assume the solute does not affect volume, which is rarely true.

Different solutes alter the volume differently depending on their density and solubility. Therefore, always consider the total final volume rather than solvent volume alone.

Ambiguity and Precision Issues with % W/V

Using percentage weight/volume has inherent ambiguities:

• “% W/V” does not specify whether the denominator refers to solvent or total solution volume explicitly.
• This leads to inconsistent interpretations in literature and lab protocols.
• Experts caution that % W/V is less precise than molarity or molality, which rely on moles or mass per true solution volume or solvent mass.
• Best practice involves stating exact preparation methods or using metric volumetric flasks to remove ambiguity.

Relationship Between % W/V and % W/W

Weight/volume percentage differs from weight/weight percentage (% W/W). However, when the solvent is water with a density close to 1 g/mL, the two values can be similar numerically.

• % W/W is the weight of solute as a percentage of the total weight of solution.
• % W/V expresses solute mass per volume of solution.
• For aqueous solutions, 10% W/V roughly corresponds to 10% W/W due to water density near 1 g/mL.
• Precision matters when exact molarity or density measurements are required.

Summary of Key Points

• 10% W/V means 10 grams of solute dissolved in enough solvent to make exactly 100 mL of total solution.
• It does not mean 10 g solute added to 90 mL of solvent.
• Adding 10 g solute to only 90 mL solvent results in a volume less than 100 mL, thus concentration is not exactly 10% W/V.
• Solute occupies volume; final solution volume differs from solvent volume.
• Always measure final volume after solute dissolves when preparing solutions.
• % W/V notation may cause ambiguity; clearly state preparation to avoid confusion.
• Molarity or molality may offer more precise concentration measures.

Does 10% W/V mean 10g of solute dissolved in 90 mL of solvent?

No, 10% W/V means 10 grams of solute in 100 mL of total solution, not just solvent. The final volume includes both solute and solvent.

How do you prepare a 10% W/V solution correctly?

Dissolve 10 g of solute in some solvent, then add more solvent until the total volume reaches 100 mL. This ensures the concentration is 10% W/V.

Why is 10 g solute in 90 mL solvent not a 10% W/V solution?

Because volume changes when solute dissolves. The total solution will be more than 90 mL, so the ratio is not exactly 10 g per 100 mL, making it less than 10% W/V.

What mass of solute corresponds to 10% W/V in 90 mL of solvent?

For 90 mL total volume, 10% W/V means 9 g of solute. Using 10 g would make concentration higher than 10% W/V.

Is % W/V concentration precise and unambiguous?

No, % W/V can be ambiguous. It is best to specify if it refers to solute per total solution volume. Molarity or molality offer more precision.