Different between solvent and solution

[Lunasly]

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Alright, I understand the definitions of each of these, but what gets me is when I see them in formulas.

For example: If I am asked to find the % solution by mass of a 1 m (molality) NaCl solution, I would need to convert the molality into from moles/kg of solute into mass solute and then divide that by the mass of the solution.

1 mole 58.5g 58.5g
------ x --------- ----> -----------
kg 1 mole 1058.5g

So as you can see, in order to find the mass percent of the solution, we divided the mass of the solute (NaCl) by the mass of the solution (mass of solute + mass of solvent).

The question I have to ask is:Why is molarity moles of solute over litres of SOLUTION (e.x what is the molarity if 10g of NaCl is placed in a 250 mL solution)? I thought solution was defined as solvent + solute. However, if we look at this question I posed in brackets, why are we just diving by the 250 mL? Isn't the 250 mL just the solvent? Shouldn't the equation for molarity therefore read moles of solute over litres of solvent?

Thanks,
Lunasly.

 

[MedPR]

Fill a 500mL flask with NaCl(aq).

How much sodium chloride solution do you have?

How much H2O(l) do you have?

 

[Lunasly]

Fill a 500mL flask with NaCl(aq).

How much sodium chloride solution do you have?

How much H2O(l) do you have?

I feel like I am going to get embarrassed by answering a question which looks simple, but is actually difficult, lol. The aqueous means that that there is NaCl in water. So there is 500 mL of water, but isn't there also a 500 mL solution of NaCl because it's however much NaCl salt (solute) within the 500mL of water?

 

[MedPR]

I feel like I am going to get embarrassed by answering a question which looks simple, but is actually difficult, lol. The aqueous means that that there is NaCl in water. So there is 500 mL of water, but isn't there also a 500 mL solution of NaCl because it's however much NaCl salt (solute) within the 500mL of water?

No, in that example there is 500mL of solution and some number less than 500mL of water.

Consider this.

Put 10mL of pure H2O into a graduated cylinder. Add NaCl(s) and stir until saturated. The level of fluid will rise above 10mL. You now have 10mL of water (since that's what you started with), but >10mL of NaCl(aq).

There's no way you can tell how much water is in a 500mL NaCl(aq) solution without knowing how much NaCl is in there.

 

[Lunasly]

Mhmm, ok that makes sense. However, but in regards to my question, what I am saying is that wen asked for molarity, they say to add x grams of NaCl salt to 250 mL of water. If I wanted the molarity, wouldn't it be the moles of the NaCl salt divided by the 250mL solvent (water)? As you said, we know how much water is present and since we haven't added any solute to it yet, shouldn't it be considered a solvent?

 

[MedPR]

Mhmm, ok that makes sense. However, but in regards to my question, what I am saying is that wen asked for molarity, they say to add x grams of NaCl salt to 250 mL of water. If I wanted the molarity, wouldn't it be the moles of the NaCl salt divided by the 250mL solvent (water)? As you said, we know how much water is present and since we haven't added any solute to it yet, shouldn't it be considered a solvent?

Water is the solvent, yes. But molarity is always moles solute over liters solution. Molality is moles solute over kilograms solvent.

I don't think they would ask you "What is the resulting molarity of NaCl if 10g of NaCl are added to 200mL of water?" If they did, they would have to give you the density of NaCl so you could figure out the total volume of solution. If given the above data they would probably ask you for the molality since you are expected to know the density of water.

 

[Lunasly]

Ah, okay I worded my question wrong then. The concentration or molarity has to be the moles divided by the litres of solution.

Thanks.

 

[MedPR]

Ah, okay I worded my question wrong then. The concentration or molarity has to be the moles divided by the litres of solution.

Thanks.

Yep, that's the definition of molarity.