TBR: Molality vs Molarity Vs Density

[JustDoMed]

Hello everyone!
I've been googling around and using SDN, but I'm still not fully grasping the relationship between molality/Molarity. And this passage was my worst so far.

In the TBR Gen Chem book under stoichiometry (#42 &43)

42)
When 1 gram of a salt are dissolved into 100mL water, the volume of the solution is greater than 100 mL but less than 101 mL. What can be said about the concentration of solution?

A:
molality of the solution is greater than the molarity of the solution; the density of the solution is greater than that of pure water.

My reasoning:
Assuming that the mols are the same, since the volume of solution is greater than 100, the mols/L then morality should decrease since it's mols/Volume. (mol/<100 g)
For molality which deals with the mols/solvent mass (which is water?)
we get 100 mL x 1g/mL = 100g thus molality = mol/100 g
Is that right?


43) An organic compound with density that is less then 1.0g/mL is added to organic liquid also with density that is less then 1.0g/mL. What can be said about the concentration of the solution ?
A:
The molality of the solution is greater then the molarity.

This one I'm confused all together with.

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[Ms Procrastinator]

Hello everyone!
I've been googling around and using SDN, but I'm still not fully grasping the relationship between molality/Molarity. And this passage was my worst so far.

In the TBR Gen Chem book under stoichiometry (#42 &43)

42)
When 1 gram of a salt are dissolved into 100mL water, the volume of the solution is greater than 100 mL but less than 101 mL. What can be said about the concentration of solution?

A:
molality of the solution is greater than the molarity of the solution; the density of the solution is greater than that of pure water.

My reasoning:
Assuming that the mols are the same, since the volume of solution is greater than 100, the mols/L then morality should decrease since it's mols/Volume. (mol/<100 g)
For molality which deals with the mols/solvent mass (which is water?)
we get 100 mL x 1g/mL = 100g thus molality = mol/100 g
Is that right?


43) An organic compound with density that is less then 1.0g/mL is added to organic liquid also with density that is less then 1.0g/mL. What can be said about the concentration of the solution ?
A:
The molality of the solution is greater then the molarity.

This one I'm confused all together with.

Hello everyone!
I've been googling around and using SDN, but I'm still not fully grasping the relationship between molality/Molarity. And this passage was my worst so far.

In the TBR Gen Chem book under stoichiometry (#42 &43)

42)
When 1 gram of a salt are dissolved into 100mL water, the volume of the solution is greater than 100 mL but less than 101 mL. What can be said about the concentration of solution?

A:
molality of the solution is greater than the molarity of the solution; the density of the solution is greater than that of pure water.

My reasoning:
Assuming that the mols are the same, since the volume of solution is greater than 100, the mols/L then morality should decrease since it's mols/Volume. (mol/<100 g)
For molality which deals with the mols/solvent mass (which is water?)
we get 100 mL x 1g/mL = 100g thus molality = mol/100 g
Is that right?


43) An organic compound with density that is less then 1.0g/mL is added to organic liquid also with density that is less then 1.0g/mL. What can be said about the concentration of the solution ?
A:
The molality of the solution is greater then the molarity.

This one I'm confused all together with.

Dude, I have trouble even saying the two words together. My tongue just twists together, especially when I try to say them fast.

 

[kellurz]

GUYS IS NOMOGRAPHY ON THE MCAT

 

[JustinM88]

My best guess for 42)


Not sure about 43 ... @aldol16 ?

 

[aldol16]

Not sure about 43 ... @aldol16 ?

This question has been answered here: Density Question Bugging the crap out of me

In short, look at the density of the solvent. It's less than 1 g/mL, meaning that the same mole amount of solvent is going to be less numerically when measured in grams than in volume. In other words, say the density is 0.8 g/mL. The same amount of solvent will be either 0.8 g or 1 mL - both correspond to the exact same mole amount. This concept is key. When you calculate molality, you're dividing by the gram (kilogram) amount. When you calculate molarity, you're dividing by the volume amount (volume of the solution, but solvent >>> solute so it's roughly this number). Since the volume amount > gram amount in numerical terms, this means that molarity will be less than molality since you're dividing by a bigger number when you calculate it.

 

[JustDoMed]

My best guess for 42)


Not sure about 43 ... @aldol16 ?

sweet! Looks about how I solved 42, now just need 43.
Thanks man!

 

[JustDoMed]

This question has been answered here: Density Question Bugging the crap out of me

In short, look at the density of the solvent. It's less than 1 g/mL, meaning that the same mole amount of solvent is going to be less numerically when measured in grams than in volume. In other words, say the density is 0.8 g/mL. The same amount of solvent will be either 0.8 g or 1 mL - both correspond to the exact same mole amount. This concept is key. When you calculate molality, you're dividing by the gram (kilogram) amount. When you calculate molarity, you're dividing by the volume amount (volume of the solution, but solvent >>> solute so it's roughly this number). Since the volume amount > gram amount in numerical terms, this means that molarity will be less than molality since you're dividing by a bigger number when you calculate it.

Yeah I saw the thread, but I couldn't completely follow their explanation.
Thanks for breaking it down with numbers! I got it!

 

[JustinM88]

This question has been answered here: Density Question Bugging the crap out of me

In short, look at the density of the solvent. It's less than 1 g/mL, meaning that the same mole amount of solvent is going to be less numerically when measured in grams than in volume. In other words, say the density is 0.8 g/mL. The same amount of solvent will be either 0.8 g or 1 mL - both correspond to the exact same mole amount. This concept is key. When you calculate molality, you're dividing by the gram (kilogram) amount. When you calculate molarity, you're dividing by the volume amount (volume of the solution, but solvent >>> solute so it's roughly this number). Since the volume amount > gram amount in numerical terms, this means that molarity will be less than molality since you're dividing by a bigger number when you calculate it.

Cool I think I understand this concept a bit better now. Now, if we increased the Temperature (causing an increase in V of solution) would that then cause the Molarity to become even smaller then the molality? On the other hand, if we decreased the Temperature low enough, could the Molarity become greater than the molality?

 

[aldol16]

Cool I think I understand this concept a bit better now. Now, if we increased the Temperature (causing an increase in V of solution) would that then cause the Molarity to become even smaller then the molality? On the other hand, if we decreased the Temperature low enough, could the Molarity become greater than the molality?

Yeah, it all depends on the exact change in density that accompanies the temperature changes.