Getting it right, and diluting it
Six things decide whether the solution you have just made really has the concentration you calculated.
- All the apparatus used must be clean
- The mass of solute must be weighed accurately
- Material clinging to the watch glass and the funnel must be washed well into the flask
- Mixing must be done by the correct method
- The final volume must be adjusted accurately
- Contamination of the solution must be prevented
Making a weaker solution from a stronger one
The concentration of a concentrated solution can be lowered by adding more solvent to it. That is a second way to arrive at a solution you want, and it has a name.
Dilution
Reducing the concentration of a solution by adding more solvent to it is called dilution.
Most of the acids in a laboratory store are concentrated acids, and the acids actually used for experiments are very often prepared by diluting them.
The one rule that is about safety, not accuracy
Acid into water. Never the other way round.
When diluting a concentrated acid, always add the acid to the water. A great deal of heat is given out when a concentrated acid is diluted, and adding water to acid instead can make the mixture spit boiling acid out of the vessel.
Why acid into water works
Adding acid slowly to a large volume of water spreads the heat through all that water, which warms gently. Adding a little water to acid puts all that heat into a few drops, which can flash into steam and throw acid at your face.