http://physics.mercer.edu/labs/manuals/manualMECHlab/errorAnalysisDensity/ErrorAnalysis.pdf
an aluminum block displaces a volume of 15.5ml water and has a mass of 40.5g. What is its density?
Why it Could be a little different from the theoretical value? (Theoretical density of aluminum is 2.70 g / cubic cm
density = mass/ volume
15.5ml of water takes up 15.5cm cubed
Experimental density = 40.5g / 15.5cm 3
=2.61 g/cm 3
Experiment values are never going to be identical to theoretical values.
For a start the answer can only be as accurate as the equipment you are using will allow.
The above web address looks at this type of error, if you are interested enough.
+118654 
