Molarity: moles of solute per litre of solution

0:00How do we say how concentrated a solution is?

0:04A volumetric flask is made to hold exactly 250.0 millilitres when filled to its line.

0:13Take 5.85 grams of sodium chloride, NaCl: the solute, the substance that dissolves.

0:21Dissolve it in the flask in some water, the solvent.

0:27Then add water until the bottom of the meniscus, the curved surface, sits on the line.

0:34After mixing, we have 250.0 millilitres of solution, an even mix of solute and solvent.

0:43Molarity is the moles of solute in each litre of solution.

0:47c is the molarity, n is the moles of solute, and V is the litres of solution.

0:55Molar mass is the mass of one mole. For NaCl it is 58.44 grams per mole, so 5.85 grams is 0.100 moles.

1:08V is 250.0 millilitres, or 0.2500 litres.

1:16So c is 0.100 moles divided by 0.2500 litres, which is 0.400 moles per litre.

1:28Careful: V is the volume of the whole solution, not of the water added.

1:34Salt in 250 millilitres of water does not make exactly 250.0 millilitres of solution, so we fill to the line.

1:45Rearranged, the formula gives moles: n equals c times V.

1:51How many moles are in a 25.0 millilitre sample, which is 0.0250 litres?

2:00So n is 0.400 moles per litre times 0.0250 litres, which is 0.0100 moles.

2:12Molarity is moles of solute per litre of solution.