Is the concentration of solids really constant?

[sgstudent]

Solids has a constant concentration as the density of the solid is fixed. However, in the pollutant standard index for particulates and for toothpaste the concentration of calcium carbonate can vary.

This seems to contradict with the statement that solids have a constant concentration as toothpastes are said to be around 20% by mass of calcium carbonate. While for the PSI, the concentration of the solids are calculated in mass of particulates/volume of container.

So why is there a contradiction for this actually?

Thanks for the help :)

 

[Borek]

If I understand correctly what you are talking about, it is pure solid that has a constant concentration, not every solid.

 

[sgstudent]

If I understand correctly what you are talking about, it is pure solid that has a constant concentration, not every solid.

Oh what do you mean by that? Because when i put a solid block of wood into water its concentration is still moles of wood/volume of wood right? But then for the toothpaste case, the concentration is now number of moles of calcium carbonate/volume of toothpaste sample. So I'm not too sure why the 'volume' represents different things now.

Sorry for being vague in the post. Hope you can help out here :) Thanks in advance for the help!

 

[Borek]

You are confusing things, but I am not yet able to see where is the confusion.

Imagine a 1L cube of pure copper - it is has a constant concentration of copper, one that can be easily calculated. No matter what you do, concentration of copper in such a cube will be always identical.

Now imagine 1L cube of a silver/copper alloy - you can easily express its composition by listing concentrations of both metals. And you can prepare many such cubes, and each can have different composition and different concentrations of copper/silver.

 

[sgstudent]

You are confusing things, but I am not yet able to see where is the confusion.

Imagine a 1L cube of pure copper - it is has a constant concentration of copper, one that can be easily calculated. No matter what you do, concentration of copper in such a cube will be always identical.

Now imagine 1L cube of a silver/copper alloy - you can easily express its composition by listing concentrations of both metals. And you can prepare many such cubes, and each can have different composition and different concentrations of copper/silver.

My confusion was the difference in concentrations. Like for a pure substance we would say that the concentration is fixed. But for a impure solid, why would the volume of the silver (in the alloy example) include the volume of the copper too? I was thinking the volume should be just of the silver only.

As for the PSI thing, it calculates the concentration of the solids in the gas. So now they take the total mass/volume of the container. So why should they consider the volume as of the container and not of just its own volume? Because if I had that 1L cube of pure copper in water, I wouldn't say its concentration is the silver's mass/volume of water+copper. The volume would just be the 1L right?

Thanks for the help

 

[Borek]

My confusion was the difference in concentrations. Like for a pure substance we would say that the concentration is fixed. But for a impure solid, why would the volume of the silver (in the alloy example) include the volume of the copper too? I was thinking the volume should be just of the silver only.

You have a homogeneous mixture, and you measure volume of the mixture, not separately volumes of the components.

As for the PSI thing, it calculates the concentration of the solids in the gas. So now they take the total mass/volume of the container.

Sure thing, they do it because that's what they are interested in - how much solid particulates are present per some volume of gas.

So why should they consider the volume as of the container and not of just its own volume?

Own volume of what? Of the solid? It would not tell anything about how contaminated the air is, and that's what we are measuring.

 

[sgstudent]

You have a homogeneous mixture, and you measure volume of the mixture, not separately volumes of the components.



Sure thing, they do it because that's what they are interested in - how much solid particulates are present per some volume of gas.



Own volume of what? Of the solid? It would not tell anything about how contaminated the air is, and that's what we are measuring.

Oh so actually how do we tell if the volume is going to be of the container? Because in the Kc, the volume of the solid is of just the solid and not including the water or solution surrounding it. While in the PSI we include the both the volume of the solid as well as the air in a given sample.

So I'm not too sure when to use the 2. Thanks again :)