10 - SOLUÇÕES - FRAÇÃO MOLAR

Summary

This educational chemistry video focuses on the concept of molar fractions in solutions, explaining how to calculate the molar fraction of both solute and solvent. The instructor demonstrates several examples, such as calculating the molar fraction of sodium bromide and glucose, and illustrates how to convert mass to moles for various substances. Emphasis is placed on understanding the relationships between moles, mass, and molar fractions, which are fundamental to solution concentration calculations. The video is a comprehensive guide to mastering molar fraction concepts in chemistry.

Takeaways

• 😀 Molar fraction (fração molar) is the ratio of the number of moles of a substance (solute or solvent) to the total number of moles in a solution.
• 😀 The formula for molar fraction of solute (X1) is: X1 = moles of solute / total moles of solution.
• 😀 The formula for molar fraction of solvent (X2) is: X2 = moles of solvent / total moles of solution.
• 😀 The sum of the molar fractions of solute and solvent equals 1, i.e., X1 + X2 = 1.
• 😀 Molar fraction can be used to easily calculate the molar fraction of one component if the other is known.
• 😀 In an example with 2 moles of solute (NaBr) and 8 moles of solvent (water), the molar fraction of the solute is 0.2 and the solvent is 0.8.
• 😀 To calculate molar fraction, you may need to convert mass into moles by dividing by the molar mass of the substance.
• 😀 For a solution of glucose and water, after calculating moles (0.5 mol glucose and 4.5 mol water), the molar fraction of glucose is 0.1, and water is 0.9.
• 😀 When given the molar fraction of one component and total moles, you can calculate the moles of the other component.
• 😀 In a more complex example, knowing the molar fraction of sodium chloride and water, you can calculate the mass of sodium chloride required using stoichiometric relationships.
• 😀 Understanding molar fraction calculations is essential for determining the concentration of solutions and analyzing their components.

Q & A

What is molar fraction (fraction molar) and how is it calculated?

Molar fraction (fraction molar) is the ratio of the number of moles of a solute to the total number of moles in the solution. It can be calculated as: X1 = (n_solute) / (n_solute + n_solvent), where n_solute is the number of moles of the solute, and n_solvent is the number of moles of the solvent.

How is the molar fraction of the solvent (X2) calculated?

The molar fraction of the solvent (X2) is calculated similarly to that of the solute. It is given by: X2 = (n_solvent) / (n_solute + n_solvent), where n_solvent is the number of moles of the solvent and n_solute is the number of moles of the solute.

What relationship exists between X1 and X2 in a solution?

Since X1 and X2 represent the molar fractions of the solute and solvent respectively, their sum is always equal to 1. Therefore, X1 + X2 = 1. If you know X1, you can easily calculate X2 using X2 = 1 - X1.

How is the molar fraction of a solute calculated in a practical example?

In the given example, to find the molar fraction of the solute, the number of moles of the solute (2 moles of sodium bromide) is divided by the total moles in the solution (2 moles of solute + 8 moles of water), which results in a molar fraction of 0.2 for the solute.

What role does the molar mass of a substance play in calculating the number of moles?

The molar mass of a substance is essential for converting mass to moles. The number of moles (n) can be calculated using the formula: n = mass (g) / molar mass (g/mol). For example, for glucose (C6H12O6), the molar mass is 180 g/mol.

How do you calculate the number of moles of water in a solution?

To calculate the number of moles of water, divide the mass of water by its molar mass. For instance, if you have 81 grams of water, you would divide this by the molar mass of water (18 g/mol) to find that there are 4.5 moles of water.

How can molar fractions be used to calculate the mass of a substance in a solution?

Once the molar fraction of a substance is known, you can use it to determine the amount of the substance in a solution. For example, if the molar fraction of a solute is 0.25, and the total moles of the solution are 5 moles, you can multiply 0.25 by the total moles to find the number of moles of the solute. Then, by multiplying the number of moles by the molar mass of the solute, you can find the mass.

What is the significance of knowing the molar fraction in a solution?

Knowing the molar fraction of solute and solvent helps in understanding the concentration and composition of the solution. It is especially useful for calculating other properties like boiling point elevation, freezing point depression, and osmotic pressure.

How would you calculate the mass of sodium chloride (NaCl) in a solution if the molar fraction is 0.8?

If the molar fraction of NaCl (X1) is 0.8, then the molar fraction of water (X2) would be 0.2. Using this information, the moles of NaCl can be calculated by multiplying the total number of moles of the solution by 0.8. Once the number of moles of NaCl is determined, the mass can be calculated by multiplying the moles of NaCl by its molar mass (58.5 g/mol).

What is the process of converting mass to moles in chemistry?

To convert mass to moles, you divide the mass of the substance (in grams) by its molar mass (g/mol). This gives you the number of moles of the substance, which is a measure of the quantity of particles (atoms, molecules, etc.) present.