General Chemistry Question

[PhantomfanMD]

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Can someone help me here? Calculate the molality of some commercial reagent from the following data:

HCl

formula weight = 36.465
density of sol'n = 1.19 g/mL
Weight % (wt by wt) = 37.2
Molarity = 12.1

I don't know what "weight %" means...thanks a lot!

 

[szhao]

if i remember correctly Weight % is the same as mass percent, which is basically if you were to measure the weight of the solution, how much of it is contributed from the HCl

its been a good 6 years since i took gen chem, sorry for not providing much information of use, however, i don't think you need that knowledge.

Molarity*x liter=moles in x liters
y=Density*10^3*x liter= weight of solution in that x liters
y*(1-%weight)=weight of the solvent in that x liter

Molarity*x/(Density*10^3*x*(1-%weight))=Molarity/(Density*10^3*(1-%weight)*10^-3)

mol/L/(g/ml*ml/L*kg/g)=mol/kg lol hey units work out so it must be right

Edit:

Molality is a intensive quanity, ie it does not scale to weight or amount, just thought you keep that in mind. though it would be easier to assume a quantity, but make sure your final solution is intensive.

 

[Arc]

Can someone help me here? Calculate the molality of some commercial reagent from the following data:

HCl

formula weight = 36.465
density of sol'n = 1.19 g/mL
Weight % (wt by wt) = 37.2
Molarity = 12.1

I don't know what "weight %" means...thanks a lot!

Yay this is the same stuff that I'm on, so cruising on SDN is helpful after all. For starters you need to assume you have 100g for the whole solution. Weight % should be clear as day to you now. Of that 100g, 37.2g of it is the HCl. After that, I'm not 100% what to do. Let me work it out and Ill get back to you.

 

[PhantomfanMD]

Yay this is the same stuff that I'm on, so cruising on SDN is helpful after all. For starters you need to assume you have 100g for the whole solution. Weight % should be clear as day to you now. Of that 100g, 37.2g of it is the HCl. After that, I'm not 100% what to do. Let me work it out and Ill get back to you.

I think I have it. Can't you just take...

Molarity/density to find the molality? Of course I'd have to convert by multiplying by 1000 to get the g/mL into kg/L then let that cancel with the numerator of moles/L to get moles/kg?

 

[novawildcat]

that would just be the inverse of the molecular weight of HCL. you want moles of HCl per kilo of solvent.


You have to be very careful in this problem with the correct units for the density, molarity, and molality

write them out.


Molarity=12.1 moles of HCl / 1 L of solution

Density=1.19 grams of HCl/1 mL of solution

molality= moles of HCl/ kilo of solvent

solvent and solution are not interchangeable.


Start with molarity and work your way towards molality units. You want to multiply molarity x density x %wt by wt ( in correct units of course).


(12.1 moles HCl/ 1L soln) x (1L soln/1.190 kg HCl) x (.372 kg HCl/1 kg solvent)= moles HCL/kg solvent.

 

[Arc]

Im back. Setting the weights equal to 100 g will yield 37.2 g of HCl , and 62.8 g of water (H20).Dividing each with their respective molar mass will yield 1.021 mols for HCl, and 3.485 mols for water. However we still need to know the mass of the water before we know the molality. To make things simple divide the mols of HCl (1.021) by the mass of water(.0628) and you get 16.26 m

I think 16.26 is the answer...Am I wrong?

 

[UIC Killa]

Can someone help me here? Calculate the molality of some commercial reagent from the following data:

HCl

formula weight = 36.465
density of sol'n = 1.19 g/mL
Weight % (wt by wt) = 37.2
Molarity = 12.1

I don't know what "weight %" means...thanks a lot!

SDN = new area for getting homework problems done when one is either confused or does not have enough time to finish...awesome!!...so whats your medical school question again? Did the mcat forum not able to provide the answers...lol

 

[Arc]

Hey, it gives me good practice.👍
Maybe it wont be a bad idea to use the MCAT forum to study for Chemistry tests.

 

[UIC Killa]

Hey, it gives me good practice.👍
Maybe it wont be a bad idea to use the MCAT forum to study for Chemistry tests.

lol...not a bad idea at all...just keep it in the guise of an "mcat related" question and your home free...why even ask a T.A. or the professor for that matter when you ask one question and like 20 ppl post their answers and methods...I should just scan my homework, place it in the forum and ask ppl to solve this "mcat" practice physics test...

 

[BNSN]

lol...not a bad idea at all...just keep it in the guise of an "mcat related" question and your home free...why even ask a T.A. or the professor for that matter when you ask one question and like 20 ppl post their answers and methods...I should just scan my homework, place it in the forum and ask ppl to solve this "mcat" practice physics test...

Physics is destroying my life. Worst class ever.