Mass Percentage, Volume Percentage, Mass by Volume Percentage — Definition

Definition

Imagine you have a glass of lemonade. How much sugar is in it? How much lemon juice? To answer these questions precisely, chemists use terms to describe the 'concentration' of a solution. Concentration simply tells us how much of a specific substance (called the 'solute') is dissolved in a total amount of mixture (called the 'solution').

When we talk about mass percentage, volume percentage, and mass by volume percentage, we're using different ways to express this concentration, often as a fraction out of 100.

**Mass Percentage ($w/w$ or $$):** This tells you the mass of the solute present in 100 units of mass of the solution. Think of it like this: if you have a 10% sugar solution by mass, it means that for every 100 grams of the total solution, there are 10 grams of sugar.

The rest, 90 grams, would be the solvent (usually water). This method is very common because mass is a fundamental property that doesn't change with temperature. It's useful for solid-in-liquid solutions, like salt in water, or even solid-in-solid mixtures.

**Volume Percentage ($v/v$ or $$):** This method is similar to mass percentage, but it deals with volumes instead of masses. It tells you the volume of the solute present in 100 units of volume of the solution.

For example, if an alcohol solution is 70% ethanol by volume, it means that in every 100 milliliters (or liters, or any volume unit) of the total solution, there are 70 milliliters of ethanol. This method is primarily used when both the solute and the solvent are liquids, like alcohol in water.

It's important to remember that volumes can sometimes be non-additive (meaning 50 mL of alcohol + 50 mL of water might not exactly equal 100 mL of solution due to intermolecular interactions), so the 'volume of solution' is measured directly, not just added up from solute and solvent volumes.

**Mass by Volume Percentage ($w/v$ or $$):** This is a hybrid method. It expresses the mass of the solute present in 100 units of volume of the solution. So, if a saline solution is 0.9% sodium chloride ($NaCl$) by mass/volume, it means that for every 100 milliliters of the solution, there are 0.

9 grams of $NaCl$. This method is extremely common in medicine and pharmacy, where dosages are often specified as a certain mass of drug per unit volume of solution (e.g., milligrams per milliliter). It's convenient because it allows for easy measurement of the solution by volume, while the active ingredient is quantified by mass, which is usually more critical for biological effects.

Unlike mass percentage, mass by volume percentage is temperature-dependent because volume changes with temperature.