Help in Understanding Mass Percent of Solutions in Analytical Chemistry

[popcornfrenzy]

How will you prepare 5% of a solution in 40 mL of water NaCl?

 

I'm quite familiar with how this goes:

5% Nacl is equal to 5g NaCl / 100g Solution based on % by mass formula.

Since 100g is equal to 100 mL

 

The answer here would be :

40 mL H2O x 5 g NaCl / 100 mL Soln = 2 g NaCl

 

It was stated by one of the posts in quora that max saturation of NaCl at room temperature is 357 mg / ml

 

Link:

https://www.quora.com/How-will-you-prepare-5-of-a-solution-in-40-ml-of-water-NaCl

 

I had a practical exam similar to this  and I horribly failed because I thought the 15% NaCl which around 36 grams of NaCl (I think, I already forgot, but this is only for context) will be deducted to 250 g H2O; therefore, I mixed 36 g NaCl and 214 g H2O in a volumetric flask to make a 250 g solution. And since 1 g = 1 ml, I made an assumption that my solution will be exactly at 250 mL too. I was so wrong, and the solution barely reached the line in the volumetric flask. I was also expecting that the resulting solution will be at max saturation, thinking that saturation was the point of the activity. 

Anyways my question is, do these kind of problem actually takes into account max saturation? Or does it only help in preparing an specific % of solute to solution ratio regardless whether the result will be unsaturated, saturated, or supersaturated? 

 

Follow up: What is the point of this activity? Like if ever that this is not about saturation then why do I have to make random concentrations base on percentage? Shouldn't a standard solution should be followed at fixed percentage? Like I would rather have a saturated solution (which already have tables in fixed values in the internet at room temp) for experiments than like use 10% NaCl, 15% NaCl or 50% NaCl.

[Bufofrog]

21 minutes ago, popcornfrenzy said:

And since 1 g = 1 ml

That is true for water, for instance 1 gram of mercury is certainly not 1 ml.

24 minutes ago, popcornfrenzy said:

Follow up: What is the point of this activity? Like if ever that this is not about saturation then why do I have to make random concentrations base on percentage?

The point is to develop your understanding solutions.

[studiot]

1 hour ago, popcornfrenzy said:

Anyways my question is, do these kind of problem actually takes into account max saturation? Or does it only help in preparing an specific % of solute to solution ratio regardless whether the result will be unsaturated, saturated, or supersaturated? 

 

Follow up: What is the point of this activity? Like if ever that this is not about saturation then why do I have to make random concentrations base on percentage? Shouldn't a standard solution should be followed at fixed percentage? Like I would rather have a saturated solution (which already have tables in fixed values in the internet at room temp) for experiments than like use 10% NaCl, 15% NaCl or 50% NaCl.

Rejoice for thou art living in the modern age.

In your last question about concentration I told you that different disciplines use different measures of concentration.

It was even more confused in oder times as chemists used equivalents and normalities,  -  you can still come across these measures.

So beware of using the term 'equivalents' or is equivalent to.

 

1 hour ago, popcornfrenzy said:

Since 100g is equal to 100 mL

How can 100 grams be equal to 100 mL ? grams are not millilitres.

 

In fact what you mean is that 100 grams occupies 100 mL or 100mL weighs 100 grams, neither of which is true except at zero concentration.

 

So please, for you furute sake, try to be more precise with your wording.

 

But, I said rejoice because solutions of sodium chloride have a density that is very close to that of water.

In fact you can get density tables and calculatiors on the internet these days to find the exact weight of 100mL of solution or the exact volume of 100g of a given %  solution.

 

https://www.handymath.com/cgi-bin/nacltble.cgi?submit=Entry

[popcornfrenzy]

31 minutes ago, studiot said:

n fact what you mean is that 100 grams occupies 100 mL or 100mL weighs 100 grams, neither of which is true except at zero concentration.

 

1 hour ago, Bufofrog said:

That is true for water, for instance 1 gram of mercury is certainly not 1 ml.

 

I actually thought that the 1 g = 1 mL is a universal thing for all solvents, glad that you guys made me realize this

 

So basically, the activity was only for making different concentrations of NaCl, nothing more than that?

[exchemist]

50 minutes ago, popcornfrenzy said:

 

 

I actually thought that the 1 g = 1 mL is a universal thing for all solvents, glad that you guys made me realize this

 

So basically, the activity was only for making different concentrations of NaCl, nothing more than that?

In any mass <-> volume conversion you always need to take into account density. The density of water happens to be 1g/ml, strictly speaking at 4C but close enough for most purposes at most temperatures.

I spent my career in the oil industry, where we handled a variety of oils all with different densities. This made it very important to use the correct figure when using volumetric meters, drum filling scales etc. and also to correct for the way density changes with temperature. Liquids tend to expand with rising temperature, so the density decreases. 

Many organic solvents have densities considerably below 1g/ml. For instance n-hexane has a density of 0.66g/ml at 25C. Others are considerably denser than water, e.g. carbon tetrachloride has a density of about 1.5g/ml.  

Edited March 21, 2023 by exchemist