Chemistry for Physiology

While “iso-” refers to the same, it is the concentration of all the dissolved particles be they ions or molecules of any substance that determines tonicity.

Mass of 1 mol of glucose = (6 × 12) + (12 × 1) + (6 × 16) = 72 + 12 + 96 = 180 g.

Although colloid osmotic pressure has the word “pressure”, this does not mean that a physical pressure is exerted on capillary walls due to the colloids (the plasma proteins).

Both mole and osmole may be used in reference to ionic compounds (because the atoms of ionic compounds separate into their constituent ions when dissolved), while the atoms within covalent compounds remain as molecules even when dissolved.

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.

This is a definition of osmosis.

Choice B is wrong if the two atoms are the same, e.g. if both are oxygen. Choice D is wrong as it implies a non-polar bond.

The pH of a buffer does change, albeit very slightly, on addition of acid or base.

A pH lower than the healthy blood pH range is called acidosis.