To prepare a 0.25 m (molal) solution of citric acid, we first need to determine the mass of citric acid required. Molality (m) is defined as the number of moles of solute per kilogram of solvent. Here, we are dealing with a 75 ml solution, which has a density close to that of water (approximately 1 g/ml), so we can assume that the mass of the solvent (water) is approximately 75 g or 0.075 kg.
Using the formula for molality:
m = moles of solute / kg of solvent
Rearranging this gives us:
moles of solute = m × kg of solvent
Plugging in the values:
moles of solute = 0.25 m × 0.075 kg = 0.01875 moles
Next, we need to find the mass of citric acid needed. The molar mass of citric acid (C6H8O7) is about 192.13 g/mol. To find the mass:
mass = moles × molar mass
mass = 0.01875 moles × 192.13 g/mol ≈ 3.61 g
Thus, you would need approximately 3.61 grams of citric acid to prepare a 75 ml solution at 0.25 m concentration.
In this solution, citric acid is the solute, and water is the solvent.
Now, regarding the concept of ‘like dissolves like’, this principle suggests that polar solvents will typically dissolve polar solutes, while nonpolar solvents will dissolve nonpolar solutes. Citric acid is a polar molecule due to its multiple hydroxyl (–OH) groups and carboxylic acid (-COOH) groups, which form hydrogen bonds with water, a polar solvent. Thus, citric acid dissolves well in water.
If we consider hexane, it is a nonpolar solvent. Given that citric acid is polar, it would not dissolve well in hexane. The lack of interaction between the polar citric acid and the nonpolar hexane leads to poor solubility, illustrating the principle that like dissolves like.