Molality quiz #1 Flashcards
Molality quiz #1
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Why do we use molality instead of molarity for certain solution concentration calculations? Molality is used because it is temperature-independent; it is based on the mass of the solvent rather than the volume of the solution, which can change with temperature. What is the molality of a solution that contains 75.3 g of KCl dissolved in 95.0 g of water? First, calculate moles of KCl: 75.3 g / 74.55 g/mol = 1.01 mol. Convert water to kg: 95.0 g = 0.0950 kg. Molality = 1.01 mol / 0.0950 kg = 10.6 mol/kg. What are the units of molality? The units of molality are moles of solute per kilogram of solvent (mol/kg). How do you determine the molality of a phosphoric acid solution if given the mass of solute and solvent? Calculate moles of phosphoric acid using its molar mass, then divide by the mass of solvent in kilograms: molality = moles of H₃PO₄ / kg of solvent. What is the molality of lithium ions in a 0.302 m solution of Li₃PO₄, assuming complete dissociation? Li₃PO₄ dissociates into 3 Li⁺ and 1 PO₄³⁻ per formula unit. Lithium ion molality = 3 × 0.302 m = 0.906 mol/kg. What is the molality of a solution prepared from 13.2 g MgCl₂ and 175.0 g H₂O? Moles of MgCl₂: 13.2 g / 95.21 g/mol = 0.139 mol. Solvent in kg: 175.0 g = 0.175 kg. Molality = 0.139 mol / 0.175 kg = 0.794 mol/kg. What is the molality of a solution prepared from 12.5 g KNO₃ and 75.0 g H₂O? Moles of KNO₃: 12.5 g / 101.1 g/mol = 0.124 mol. Solvent in kg: 75.0 g = 0.0750 kg. Molality = 0.124 mol / 0.0750 kg = 1.65 mol/kg. How does the calculation of osmolality differ for covalent and ionic compounds? For covalent compounds, osmolality equals the molality since they do not dissociate; for ionic compounds, osmolality is the number of ions produced multiplied by the molality. What is the symbol used to represent molality, and why is it significant? Molality is represented by a lowercase 'm', which distinguishes it from molarity and emphasizes its temperature independence. If a solution contains 2.5 mol of NaCl, what is its osmolality and why? NaCl dissociates into 2 ions, so osmolality is 2 × 2.5 mol = 5 mol; this accounts for all dissolved particles in the solution.
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