Solutions and Concentration Flashcards

A student needs to prepare 500.0 mL of a 0.500 M sodium chloride (NaCl) solution. How many grams of NaCl are required? (Molar Mass of NaCl ≈ 58.44 g/mol)

Answer: 14.6 g

To find the mass of NaCl needed, first calculate the number of moles required using the molarity formula: Moles = Molarity × Volume (L). Moles = 0.500 mol/L × 0.5000 L = 0.250 moles. Then, convert moles to grams using the molar mass: Mass = Moles × Molar Mass. Mass = 0.250 mol × 58.44 g/mol = 14.61 g, which rounds to 14.6 g.

If 25.0 mL of a 4.00 M stock solution of HCl is diluted with water to a final volume of 200.0 mL, what is the molarity of the resulting diluted solution?

Answer: 0.500 M

This is a dilution problem that can be solved using the formula M₁V₁ = M₂V₂, where M₁ and V₁ are the initial molarity and volume, and M₂ and V₂ are the final molarity and volume. (4.00 M)(25.0 mL) = M₂(200.0 mL). Solving for M₂ gives: M₂ = (4.00 M × 25.0 mL) / 200.0 mL = 0.500 M.

Which of the following concentration units is temperature-dependent because it is based on the volume of the solution?

Answer: Molarity (M)

Molarity is defined as moles of solute per liter of solution. Since the volume of a solution can change with temperature (expand when heated, contract when cooled), molarity is a temperature-dependent unit. Molality, mass percent, and mole fraction are all based on mass or moles, which are not affected by temperature changes.

A solution is prepared by dissolving 20.0 g of glucose (C₆H₁₂O₆) in 80.0 g of water. What is the mass percent of glucose in this solution?

Answer: 20.0%

Mass percent is calculated as (mass of solute / mass of solution) × 100%. The mass of the solute is 20.0 g of glucose. The mass of the solution is the mass of the solute plus the mass of the solvent (20.0 g + 80.0 g = 100.0 g). Therefore, the mass percent is (20.0 g / 100.0 g) × 100% = 20.0%.

A chemist dissolves 58.44 g of NaCl (1 mole) in 500 g of water. Which of the following correctly describes the concentration of this solution?

Answer: 2.0 molal

Molality (m) is defined as moles of solute per kilogram of solvent. First, convert the mass of the solvent (water) from grams to kilograms: 500 g = 0.500 kg. Then, calculate the molality: Molality = moles of solute / kg of solvent = 1 mole NaCl / 0.500 kg H₂O = 2.0 m.

To increase the concentration of a saturated aqueous salt solution at a constant temperature, a student should:

Answer: Evaporate some of the water.

In a saturated solution, the maximum amount of solute is already dissolved. Adding more solute will not increase the concentration. Adding more solvent (water) will decrease the concentration (dilution). Stirring only speeds up the rate of dissolving but doesn't change the saturation point. Evaporating the solvent (water) will remove it from the solution, thereby increasing the ratio of solute to solvent and thus increasing the concentration.