[danavuia.1987]

How do i prepare H2SO4 1:3?
Why 50 ml H2So4 +150 H2O does not result 200 ml of solution H2SO4 1:3?

basically results aprox 170 ml of H2SO4 1:3.
Waiting for an answear.

[Borek]

How do i prepare H2SO4 1:3?

By mixing 1 volume of teh acid with three volumes of water.

Why 50 ml H2So4 +150 H2O does not result 200 ml of solution H2SO4 1:3?

Volumes are not additive, the effect is called volume contraction of liquids (google it).

[Arkcon]

Depending on the application, you may or may not have done it right.

Like Borek: said, liquid volumes may or may not be additive.  That is simply the way the physical chemistry works.  The effect may be negligible, or significant, and which can depend on the application.

If a solution has a change in temperature on mixing, the volume can also change.  And again, this can be significant, or not depending on the application.  Funny, I'd expect mixing sulfuric and water to get very hot, and increase in volume slightly, then contract as cooled.

Hrm ... did you mix 50 ml sulfuric and 150 ml water, measured separately and then mixing, to get your solution?  Because that's how I'd do it.  Or did you mix them in one measuring vessel, and then let it cool, because that's what would give what you've seen.

The protocol you're using should tell you exactly how its done.  This is definitely the case for a pharmacopeia method, may or may not be the case for ASTM or other methods.

Possibly, its not a significant part of the assay, 1:3 is a pretty general sort of solution, maybe this reagent is used, in an assay, to just keep the pH low?  You can describe the assay for us better so we can be sure.

If this is a titrant, or other quantitative reagent, then it should be defined better in the procedure, and prepared by you more carefully, you can let us know about that as well.

[danavuia.1987]

I have measured in 2 separated cylinder : in first 50 ml H2SO4 98% and in second 150 ml distilled water and then i put the 50 ml H2SO4 98% in the 150 ml distilled water  and read the level:around 170 ml.

[Borek]

I have measured in 2 separated cylinder : in first 50 ml H2SO4 98% and in second 150 ml distilled water and then i put the 50 ml H2SO4 98% in the 150 ml distilled water  and read the level:around 170 ml.

Something is definitely wrong here, while the contraction is something to be expected in this case the final volume should be around 190 mL.