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# Calculating Solution Concentration ## Introduction We've used molarity (M) to express solution concentration. However, molarity is not ideal for precise measurements because volume is temperature-dependent. Changes in volume affect concentration. Therefore, we need alternative methods less sus...

# Calculating Solution Concentration ## Introduction We've used molarity (M) to express solution concentration. However, molarity is not ideal for precise measurements because volume is temperature-dependent. Changes in volume affect concentration. Therefore, we need alternative methods less susceptible to environmental factors. ## Molality (m) Molality (m) is the moles of solute per kilogram of solvent. It's a temperature-independent measure. * **Formula:** m = moles of solute / kg of solvent ## Mole Fraction (X) Mole fraction (X) is a direct measure of the ratio of the number of molecules of a component to the total number of molecules present in the solution. It's useful for studying the effects of solute molecules on solvent molecules. * **Formula:** XA = (moles of A) / (total moles) ## Parts by Mass (m/m) Parts by mass (m/m) is used to compare different concentrations quickly. * **Formula:** m/m = (mass of solute) / (total mass of solution) ## Example Calculation **Problem:** Calculate the molality, mole fraction, and mass fraction of a solution containing 80.9 g of HBr and 1.00 kg of H₂O. **Solution:** * **Molality:** 1. Calculate moles of HBr: 80.9 g HBr / 80.9 g/mol HBr = 1.00 mol HBr 2. Calculate molality: 1.00 mol HBr / 1.00 kg H₂O = 1.00 molal * **Mole Fraction:** 1. Calculate moles of water: 1.00 kg H₂O / 18.015 g/mol H₂O = 55.5 mol H₂O 2. Calculate mole fraction of HBr: 1.00 mol HBr / (1.00 mol HBr + 55.5 mol H₂O) = 0.0177 * **Mass Fraction:** 1. Calculate the total mass of the solution: 80.9 g HBr + 1000 g H₂O = 1080.9 g 2. Calculate the mass fraction of HBr: 80.9 g HBr / 1080.9 g solution = 0.0749 3. Expressing this in percent form will be 7.49% **Important Notes:** * Different concentrations (like molality, mole fraction, parts by mass) are affected differently by the presence of various solutes. For instance, while mole fraction of HF, HCl, and HBr solutions in water have similar mole fractions, their part by mass values will be different. * Parts by mass relates to percent mass by setting the denominator as 100%. * For example, if a solution has 25 g of solute and 100 g of solution, the mass fraction of the solute is 25/100 = 25%. * ppm (parts per million) or ppb (parts per billion) are expressions of concentration by parts and are applied on smaller concentrations. ppm is obtained from multiplying (mass fraction) by $10^6$ and ppb is obtained from multiplying (mass fraction) by $10^9$.