[frezhhigh]

A 75.0 gram of pure liquid with a density of 3.00g/mL is mixed with a 50.0 mL of pure liquid with a density of 2.00g/mL what is the total volume of the mixture

[Borek]

No answer to this question, volumes are not additive.

You are probably expected to assume final volume is a sum of volumes, but it is not always truth. 100 mL of water mixed with 100 mL of ethanol gives about 193 mL of mixture.

[Schrödinger]

@ Borek : I have heard that volumes are not additive. What's the reason behind this?
It's understandable for mixtures that chemically react with each other to produce  products that separate out of the solution.
But the example you gave....water + ethanol...Why is there volume reduction in this case?

I thought about exothermicity of the reaction, but none of the liquids boils and goes out as vapour right? (Or maybe that's what I know)

[sidharth]

@Schrödinger : this would be my tentative answer and I am not sure that it would be correct ----
this has got to do something with molecular rearrangement due to new molecular interactions such hydrogen bonding in the case of mixing alcohol with water which leads to molecules coiming closer together thus reduction in the volume of the solution formed

[Borek]

Imagine mixing sand and peas - sand occupies volume between peas, so the final volume is lower than sum of the volumes. That's not the only reason, but whenever there are molecules that differ by size it plays a role.