[Needaask]

When asked to find how much of the solvent is in the v/v, we could just find the volume of solution minus the volume of solute to get it. Why is this so? Are they assuming that the volume is additive for the compounds?

And for w/w the same could be said for the mass of the solvent. And I guess this makes sense as mass is definitely additive.

However, for w/v where the solvent is water, can we do anything here? I'm thinking if I know the volume of the solution and mass of the solute, I still wouldn't be able to find anything unless they gave us the density of one of those 2. Because if I'm given the density of the solution, I can find the mass of solution and hence I can get the mass of the solvent. And if I'm given the density of the solute then I can get the volume of the solute and hence I can get the volume of the solvent. Am I right about this?

Thanks

[Needaask]

I thought about this again. If they gave us the density of one of the 2 is good enough because we already know that the density of water is 1g/ml.

So if we have the density of the solute, we can find the volume of the solute and then from here we can use the volume of the solution to get the volume of the water. And we can convert that to grams using the 1g/ml density of water.

And if we have the density of the solution, we can find the mass of the solution and using that, we can minus that off the mass of solute. And we would get our mass of water alone. And again we can find convert the mass into volume using the 1g/ml density.

But in these cases we have to assume that volume is additive right?

[Dan]

Can you post specific examples? This will make your question much easier to answer.

Assuming volume is additive is not always a good assumption - mass is additive though.

[Needaask]

Can you post specific examples? This will make your question much easier to answer.

Assuming volume is additive is not always a good assumption - mass is additive though.

The example for additive volume was alcohol and blood.

While the example the teacher gave for the w/v was this: 0.92g of NaCl and 99.08ml of water mixed to form 100ml of solution.

But I'm not sure that this is correct because the density of sodium chloride is not 1g/cm3 so they shouldn't add up to form 100ml of water right? I would need the density of the NaCl to get the volume of it to find my total V right?

[Borek]

While the example the teacher gave for the w/v was this: 0.92g of NaCl and 99.08ml of water mixed to form 100ml of solution.

Assuming 20°C 99.08 mL of water weights 98.90g, so the total weight of the solution is 99.82 g. 0.92 g of NaCl in 99.82 g of solution is 0.922% w/w, and teh density is 1.0047 g/mL. That yields a volume of 99.35 mL.

Assuming (as intended) that the mass of the solution is exactly 100 g, change sthe result slightly, but mostly because of the additional mass. Concentration is now 0.920% w/w, density again 1.0047 g/mL, final volume 99.53 mL.

For most applications 99.35 and 99.53 are close enough to 100, for some they are not.

Sm stands for solution mass.

[Needaask]

While the example the teacher gave for the w/v was this: 0.92g of NaCl and 99.08ml of water mixed to form 100ml of solution.

Assuming 20°C 99.08 mL of water weights 98.90g, so the total weight of the solution is 99.82 g. 0.92 g of NaCl in 99.82 g of solution is 0.922% w/w, and teh density is 1.0047 g/mL. That yields a volume of 99.35 mL.

Assuming (as intended) that the mass of the solution is exactly 100 g, change sthe result slightly, but mostly because of the additional mass. Concentration is now 0.920% w/w, density again 1.0047 g/mL, final volume 99.53 mL.

For most applications 99.35 and 99.53 are close enough to 100, for some they are not.

Sm stands for solution mass.

Hi thanks for the detailed calculations

Actually why would the density of both solutions be the same?

So in some cases we can just add the masses of the water and the solute to give the volume of the solution? Wouldn't it be more accurate to get both volume via their densities. Then adding them up to get the volume of solution?

[Borek]

Densities are not exactly the same, but the difference is below the accuracy of the tables. Even values given are estimated from known neighbors assuming linear dependency.

Best approach is to convert volumes to masses using densities, than to add masses, then to convert the final mass to volume using density again. You need several densities for that, but as long as you are using masses and you have density tables in the form density vs % w/w it is relatively easy.

Not that I am doing it manually, that's what the concentration converter exist for.

[Needaask]

Densities are not exactly the same, but the difference is below the accuracy of the tables. Even values given are estimated from known neighbors assuming linear dependency.

Best approach is to convert volumes to masses using densities, than to add masses, then to convert the final mass to volume using density again. You need several densities for that, but as long as you are using masses and you have density tables in the form density vs % w/w it is relatively easy.

Not that I am doing it manually, that's what the concentration converter exist for.

Yeah that makes the most sense because using volume isn't right too cos' they aren't additive as you mentioned in the other thread.

So if questions don't give the densities how should i approach them?

And if they only gave one density eg of the solution, then if I'm asked to find the mass of solvent then I can just find the total mass of solution(by density)-mass of solute to get my mass of solvent right?

And if they only gave the density of the solute instead, then I would only be able to find its volume so I'd have to assume that the volume is additive here? So I'd get the volume of solute and use volume of solution-volume of solute to get the volume of solvent. And since the solvent is water i can get the mass by the 1g/cm3 density of water?

So actually the best would be to use mass to make sure everything is additive? But what if i only have the solute and solution's respective volumes? And lastly, if I only have the solute's mass and solution's volume?

Thanks for the help Borek

[Borek]

So if questions don't give the densities how should i approach them?

Most often it is enough to assume additiveness of the volumes and density of the solution to be 1 g/mL. Never perfect, but usually good enough.

And if they only gave one density eg of the solution, then if I'm asked to find the mass of solvent then I can just find the total mass of solution(by density)-mass of solute to get my mass of solvent right?

Yes.

And if they only gave the density of the solute instead, then I would only be able to find its volume so I'd have to assume that the volume is additive here? So I'd get the volume of solute and use volume of solution-volume of solute to get the volume of solvent. And since the solvent is water i can get the mass by the 1g/cm3 density of water?

Density of the solute is usually useless. Most solutes are solids, densities are given for the crystal form, bulk substance has different density, as large part is the air between grains. These questions - unless there is some additional information given - are usually impossible to solve.

[Needaask]

Thanks for the reply this cleared up a lot for me. Right now I'm able to do the tutorial questions for this topic but I'm still quite scared for these types of questions.

So in this case my teacher's working is correct where 0.92g of NaCl and 99.08g of water forms 100ml of solution right? Because the 100ml is about 100g. So if they were to leave the volume of water a variable, I would just use 100g minus the mass of NaCl which would give me 99.08g? Would this mean that the density of NaCl is also 1g/cm3?

Thanks again

[Borek]

So in this case my teacher's working is correct where 0.92g of NaCl and 99.08g of water forms 100ml of solution right?

0.92 g of NaCl and 99.08 g of water gives exactly 100 g of the solution, but the volume will be slightly different from 100 mL (I already posted exact volume earlier). In most cases the difference doesn't matter, so the assumption that there is 100 mL of the solution is good enough, but assuming it is 100 mL and knowing it is 100 mL are two different things.

Because the 100ml is about 100g. So if they were to leave the volume of water a variable, I would just use 100g minus the mass of NaCl which would give me 99.08g?

Correct enough, and in most cases it is the best approach to be taken.

Would this mean that the density of NaCl is also 1g/cm3?

No, it doesn't say anything about the solid density.

[Needaask]

So in this case my teacher's working is correct where 0.92g of NaCl and 99.08g of water forms 100ml of solution right?

0.92 g of NaCl and 99.08 g of water gives exactly 100 g of the solution, but the volume will be slightly different from 100 mL (I already posted exact volume earlier). In most cases the difference doesn't matter, so the assumption that there is 100 mL of the solution is good enough, but assuming it is 100 mL and knowing it is 100 mL are two different things.

Because the 100ml is about 100g. So if they were to leave the volume of water a variable, I would just use 100g minus the mass of NaCl which would give me 99.08g?

Correct enough, and in most cases it is the best approach to be taken.

Would this mean that the density of NaCl is also 1g/cm3?

No, it doesn't say anything about the solid density.

Oh but since we assume that the solution's density is 1g/cm3 won't we have to assume that the density of solid NaCl is also 1g/cm3 too?

[Borek]

Oh but since we assume that the solution's density is 1g/cm3 won't we have to assume that the density of solid NaCl is also 1g/cm3 too?

No. Besides, it doesn't matter.