Making 500 cubic centimetres

One complete preparation, worked out and then carried out: 500 cm³ of sodium chloride solution at 1 mol dm⁻³.

First, the mass to weigh out

The calculation

molar mass of sodium chloride = (23.0 + 35.5) g mol^-1

= 58.5 g mol^-1

mass of NaCl in 1000 cm³ of a 1 mol dm^-3 solution = 58.5 g

mass of NaCl in 500 cm³ = (58.5 g ÷ 1000 cm³) × 500 cm³

= 29.25 g

Then, the preparation

Figure to be added

A hand squeezing a wash bottle to rinse weighed sodium chloride off a watch glass, through a funnel, into the neck of a 500 cm³ volumetric flask standing on the bench.

Weigh 29.25 g of sodium chloride very accurately onto a watch glass, using a laboratory balance.
Choose a clean volumetric flask marked at 500 cm³.
Remove its stopper and hold a clean funnel in the neck.
Wash the whole of the weighed sodium chloride off the watch glass and into the funnel with the wash bottle. Then wash down the inner surface of the watch glass and the inner surface of the funnel into the flask as well.
Add about two thirds of the water needed and stopper the flask.
Mix well so that all the sodium chloride dissolves.
Once it has all dissolved, with your eye level with the mark on the neck, add water carefully until the meniscus is set as in the figure below. Stop adding water there.
Stopper the flask and mix thoroughly again.

Reading the mark

Figure to be added

A close view of the neck of a volumetric flask, with the curved lower surface of the liquid — the meniscus — sitting exactly on the etched graduation mark, viewed at eye level.

The line on the neck is read against the bottom of the curved liquid surface, with your eye at the same height as the line. Looking down on it or up at it puts the reading out, and at this stage an error of a millimetre in the neck is an error in the concentration.

Two thirds first, and mix before filling

The solid is dissolved in about two thirds of the water and only then made up to the mark. Fill to the mark first and the solid may not dissolve without stirring that pushes the level past it — and once you are over the line the whole preparation has to start again.