From Molarity to Percent Composition

Molarity and percent composition describe the concentration of a solution or the composition of a compound. Molarity focuses on the concentration of a solution, while percent composition describes the elemental makeup of a compound.

🧪 Methods of Expressing Concentrations

Molarity (M)

Molarity measures of the concentration of a solute in a solution.

Molality (m)

Molality measures the concentration of a solute in a solution in terms of the number of moles of solute per kilogram of solvent.

Here’s an example. Imagine holding a bag containing exactly 1 kilogram of ice (our solvent). If we add 2 moles of salt (solute) to this ice, we have created a 2 molal solution.

Percent Composition

The percent composition of a compound refers to the percentage by mass of each element in the compound.

📍 By Mass

• Definition: The ratio between the mass of the solute and the total mass of the solution.
• Formula: Percent composition= (Molar mass of compound)/(Mass of element)×100%
  • Mass of element: the mass of the particular element present in the compound
  • Molar mass of compound: the total molar mass of the compound
• Application: Helps determine what proportion of a compound's mass comes from each element.

📍 By Volume

• Definition: The ratio between the volume occupied by the component (solute) and the total volume.
• Formula: Percent Composition= (Total volume of mixture)/(Volume of component) ×100%
• Application: It calculates the proportion of a specific component's volume relative to the total volume of the mixture.

Example:

Image of chocolate bar with nuts.

Image Courtesy to Pexels.

For percent by mass, envision a chocolate bar with nuts. If it says "70% Cocoa," that means out of every 100 grams, there are 70 grams worth just cocoa. For percent by volume, think about a bottle labeled "5% Alcohol." This means if you had 100 milliliters from this bottle, alcohol would be just five milliliters.

📐 Practice Problems

Now let's practice some calculations:

Preparing Solutions:

1. To make a 1 M sugar water solution, how many moles do I need for half a liter?

   Explanation: To make a 1 M of sugar water solution, you need to dissolve one mole of sugar in one liter of water. However, you want to make only half a liter of the solution, so you'll need half the amount of sugar.

   Since the solution is 1 M, it means there is 1 mole of sugar per liter of solution. Half a liter is 0.5 liters, so you'll need half the amount of sugar required for one liter.

   Number of moles=1  mol/L ×0.5 L = 0.5  moles

Percent Composition Adjustments: 3. If your lotion contains 60 g glycerin in 150 g mixture, what’s its percent composition by mass?

Explanation:

The lotion contains 60 g of glycerin and the total mass of the mixture is 150 g.

To find the percent composition by mass of glycerin in the lotion, you can use the formula:

Percent composition(%)= (Total mass of mixture/ Mass of glycerin) ×100%

                                = (60g/ 150g) ×100%

                          = (2/5) ×100%

                    =  40%

The percent composition by mass of glycerin in the lotion is 40%.

✨ Tips for Success

Here are some tips to help you master these concepts:

• Always keep track units; they can guide your calculations correctly.
• Remember molarity involves liters not kilograms—don't mix them up!
• Temperature may affect volumes; molality doesn’t care about temperature because it’s based on mass.
• When solving problems involving dilutions or percent compositions, write down what each value represents—don't get lost in numbers!
• Practice makes perfect! Try different problems to get comfortable with conversions between units and types.

So, molarity is the concentration of a solution while percent composition is the relative mass or volume of each component in a mixture. Both concepts are important in chemistry for accurately measuring and controlling the amount of substances in a solution. Good luck and happy studying! 🥳