1. One mole of ANY compound equals Avogadro’s Number of molecules (i.e., 6.023 x 1023).
2. Therefore, one mole of sodium chloride (molecular weight: 58.44 g/mol) and one mole of sodium hydroxide (molecular weight: 40 g/mol) have different molecular weights (because they contain different elements), but one mole of each consists of identical number of molecules.
3. Concentration = Amount per unit volume (Examples: g/ml, g/L, moles/L, Molar, ng/ml, ng/μl, % (weight/volume or % (w/v), % (volume/volume or % (v/v) etc…
4. The term one molar (1 Molar or 1 M) means one thing, and one thing only……….one mole per litre. In other words, the “AR” always signifies “per litre”. Therefore, 0.5 M means 0.5 moles per litre, 0.001 M means 0.001 moles per litre etc..
5. Students often wonder how you can have 1 ml of something at a concentration of 1 M, if 1 M is “per litre”.
Easy, think of it this way: If I make up 1 L of 1 M sodium chloride, I can dissolve 58.44 g in 1 litre, or I can dissolve half the amount (29.22 g) in half the volume (500 ml), a quarter the amount in quarter the volume, and so on. In either or any case, I still have a 1 M solution.
6. Students also wonder if the have a 1 M solution, and you withdraw a small volume from that solution, then how come it’s still the same concentration?
Easier still….think of it this way: If you have a bottle of vodka (750 ml and 40 % proof (i.e., 40 g ethanol per 100 ml)) and you pour 4 (small!) glasses of 15 ml each for you and your friends, you all get the same 40 % proof concentration. Identically, if you have “X” volume of a molar stock solution, and you withdraw a small volume from it, the concentration is identical to the stock.
7. % (weight/volume) means weight in grams per 100 ml. Therefore, 1 % (w/v) means 1 g dissolved in 100 ml.
8. % (volume/volume) means volume in millilitres per 100 ml. Therefore, 1 % (v/v) means 1 ml made up to 100 ml, with diluent (e.g., water).