If there was water left over in my pycnometer when an unknown liquid was then put into it, what happens to the density of the unknown?

Question

This is a question on my post lab asking me to describe what would happen if there was an error during the experiment. Would the density of the unknown be less dense or more dense and why? I seriously don't know, and can't find an explanation online.

Answer 1

Hello Sage, thanks for your question!

Density is mass per volume. A pycnometer has a fixed volume. Density is determined by measuring the mass of a compound in that fixed volume, then dividing the mass you obtain by the volume of the pycnometer.

If there is a certain volume of water already in the pycnometer and then you fill up the rest of the pycnometer with your unknown compound, when you take the mass, some of that mass is due to the water and some is due to the unknown compound.

The ultimate effect on the measured density of the unknown would depend on whether the unknown compound is more or less dense than water. If the unknown compound is denser than water, then the presence of some water would result in a lower density. If the compound is less dense than water, then the presence of water would result in a larger density.

For example:

let's say there is 1ml of H2O left in a 50 ml pycometer.

Therefore, the unknown compound would take up 49ml.

H2O has a density of 1g/ml. So, H2O in the mixture has a mass of 1g.

1 ml water * 1g/ml water = 1 g

if the unknown compound has a larger density than water, eg 2.00g/ml.

49ml * 2g/ml = 98g mass unknown

Total mass= 1 g H2O + 98g unknown = 99 g total

Total volume = 1ml H2O + 49 ml unknown = 50 ml total

Total density = 99g/50ml = 1.98g/ml

This is less than the actual unknown density of 2.00 g/ml

If the unknown compound had a smaller density than water, eg 0.50 g/ml,

49ml * 0.50g/ml = 24.5 g mass unknown

Total mass=1 g H2O + 24.5 g unknown =25.5 g total

Total volume = 1ml H2O + 49 ml unknown = 50 ml total

Total density = 25.5g/50ml = 0.51 g/ml

This is more than the actual unknown density of 0.50 g/ml

Now, this is a very simplistic view. In reality, molecules in mixtures orient themselves differently than they do in pure solutions, So, density will change once two substances mix. This is why volumes are not additive (eg 5ml H2O + 10 ml acetone does not equal 15 ml mixture, because the distance between water and acetone is different than the distance between water and water or the distance between acetone and acetone).

I hope that helps.