Why must you weigh mixture before it dissolves?

[jonny008]

This is from a post lab question.

 

The lab was really easy. Basically we were graphing the relation of the density (y axis) of salt water and the concentration of salt in water (x axis)

 

As you can imagine our graph looks like a relatively straight slope with it's intercept at 1 g/ml

 

 

 

In one part of the test we had to weigh the beaker + water + salt combination SPECIFICALLY before mixing and dissolving the salt.

 

The post lab question essentially asks me why must we weight this combination before it becomes dissolved.

 

I don't get that, the weight doesn't change, it's still the same.

 

Would our graph be any different had we mixed it before weighing?

 

The only reason I can come up with is that it's technically not "salt" anymore but Na cations and Cl anions, but that seems kind of a longshot

[studiot]

Perhaps they are testing your resolve and understanding?

 

One purpose of an experiment is to test the truth or otherwise of an assertion or hypothesis.

 

You asserted

 

 

 

I don't get that, the weight doesn't change, it's still the same.

 

Did your variation prove this assertion or disprove it?

[jonny008]

 

Perhaps they are testing your resolve and understanding?

 

One purpose of an experiment is to test the truth or otherwise of an assertion or hypothesis.

 

You asserted

 

 

Did your variation prove this assertion or disprove it?

Well, that's the thing.

 

Within the procedures manual it was clearly stated that we had to weigh first, then dissolve, and that pretty much seals the deal to make me think this isn't a trick question.\

 

Besides, the question was formulated as a "Why must you weight before mixing" instead of a "Must you weight before mixing", kind of implies there is not room of argument about this point.

 

I can't really prove right or wrong because I didn't specifically weight and record the before and after mixing weights, but come on, that's common sense.

Just because it's dissolved it doesn't become lighter.

[StringJunky]

Perhaps that's the standard protocol you have to follow because with some reagents mass may be lost through evolution of gases when mixed.

[studiot]

 

 

but come on, that's common sense.

 

Maybe but it was 'common sense' that large heavy balls fall more quickly than small light ones until someone (Do you know who?) actually tried it.

 

String Junky has a good point though you might need to capture and weigh the lost gas in this other experiment to complete the law of mass conservation.

 

I also wondered if the instruction applied specifically to your first measurement, then you addded a specific amount (weight) of salt each time to the solution.

You would need the beaker and solvent weight at some point for your calculations.

 

Posting the full instructions may help someone spot a clue as to the true intentions of your examiners.

Edited September 21, 2016 by studiot

[jonny008]

 

Maybe but it was 'common sense' that large heavy balls fall more quickly than small light ones until someone (Do you know who?) actually tried it.

 

Strink Junky has a good point though you might need to capture and weigh the lost gas in this other experiment to complete the law of mass conservation.

 

I also wondered if the instruction applied specifically to your first measurement, then you addded a specific amount (weight) of salt each time to the solution.

You would need the beaker and solvent weight at some point for your calculations.

 

Posting the full instructions may help someone spot a clue as to the true intentions of your examiners.

Here's the instructions. The relevant parts are 3-7.

 

The objective of this lab was to determine the Mass% of a liquid with an unknown quantity of salt by using the calibration curve we generated.

 

[studiot]

Well that says it all then.

 

You pre weighed the second beaker accurately because you did not add an exactly known weight of salt.

You then weighed the thing again to obtain the exact salt weight.