Chemistry for Physiology
Consider a volume of 100 ml of each of the four solutions below. Which would contain the greatest number of osmoles of dissolved particles? (FYI the atomic masses of H = 1, C = 12, O = 16, Na = 23, Cl = 35.5)
5% ethanol: C2H6O
5% glucose: C6H12O6
5% sodium chloride: Na+ Cl−
10% glucose: C6H12O6
5% sodium chloride: Na+ Cl−
By adding the masses of the atoms in the formula, we can determine that 1 mol of each substance has a mass of 46 g (for ethanol: 2 × 12 + 6 × 1 + 1 × 16 = 46), 180 g (glucose) and 58.5 g (NaCl). “5%” means 5 g of substance (much less than 1 mol of each substance) is dissolved in 100 ml of solution. An ethanol molecule is much less than half the mass of a glucose molecule, so there are more than twice as many ethanol molecules as glucose molecules in a 5% solution. Hence the ethanol solution has more osmoles than either glucose solution. While NaCl has a greater formula mass than ethanol, NaCl is an ionic substance, so it separates into Na ions and Cl ions when dissolved. This doubling in the number of particles when NaCl dissolves means that choice C contains the greatest number of osmoles. Or by calculation, the number of osmoles in each 100 ml of solution is determined by dividing the mass of dissolved substance by the mass of 1 mol of that substance for 5% ethanol, 5/46 = 0.109; 5% glucose, 5/180 = 0.0278; NaCl, (5/58.5) × 2 = 0.171; and 10% glucose, 10/180 = 0.0556 osmoles per 100 ml.
5% sodium chloride: Na+ Cl− By adding the masses of the atoms in the formula, we can determine that 1 mol of each substance has a mass of 46 g (for ethanol: 2 × 12 + 6 × 1 + 1 × 16 = 46), 180 g (glucose) and 58.5 g (NaCl). “5%” means 5 g of substance (much less than 1 mol of each substance) is dissolved in 100 ml of solution. An ethanol molecule is much less than half the mass of a glucose molecule, so there are more than twice as many ethanol molecules as glucose molecules in a 5% solution. Hence the ethanol solution has more osmoles than either glucose solution. While NaCl has a greater formula mass than ethanol, NaCl is an ionic substance, so it separates into Na ions and Cl ions when dissolved. This doubling in the number of particles when NaCl dissolves means that choice C contains the greatest number of osmoles. Or by calculation, the number of osmoles in each 100 ml of solution is determined by dividing the mass of dissolved substance by the mass of 1 mol of that substance for 5% ethanol, 5/46 = 0.109; 5% glucose, 5/180 = 0.0278; NaCl, (5/58.5) × 2 = 0.171; and 10% glucose, 10/180 = 0.0556 osmoles per 100 ml.
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What distinguishes an osmole from a mole?
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Glucose is a sugar with a molecular formula C6H12O6. Given that the atomic weights are C = 12, H = 1 and O = 16, what is the mass of 1 mol of glucose?
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