What is the molality of a solution containing 11.1 g Na2SO4 dissolved in 2269.2 g of water?
Didn't I do one just like this for you.
m = mols solute/kg solvent
so it would be 11.1 divided by 2269.2 times 100?
No. m = mols solute/kg solvent. molsl Na2SO4 is not 11.1. The problem lists that as grams. and 2269.2 is not kg. I didn't write 100 anywhere in my instructions. You're not thinking straight.
so 1 divided by 2.2692
Yes, that is kg solvent. Now convert 11.1 g Na2SO4 to mols. mols = grams/molar mass = ?
is that .44068?
No. What's the molar mass of Na2SO4?
oh 142.04
ok. Now what is 11.1 grams Na2SO4 converted to mols?
isnt that 1?
Of course not. 11.1/142 is 1?
oh .07817
OK. So mols = grams/molar mass
mol Na2SO4 = 11.1 g/142.04 = 0.0782
Now m = mols/kg solvent.
you know mols = 0.0782
You know kg solvent = 2.269
So m = ?
.0345
Yes. What were you doing wrong and why did this take so long?
im not actually suure im just not the best at chemistry and was very confused
okay now last question..i didnt understand how you did this one (What are the molality and mole fraction of solute in a 25.5 percent by mass aqueous solution of formic acid (HCOOH)?
Sometimes students type in the wrong numbers in the calculator. Sometimes they misread the atomic masses and/or the molar masses of atoms/molecules. But most of the time they don't think clearly about what they are doing.
When I say m = mols/kg solvent you should recognize that the Na2SO4 was given in grams so you need to convert to mols. mols always = grams/molar mass so you know to 11.1/142 = ?
Then since m = mols/kg solvent, you cnvert 2269 grams solvent to 2.269 and divide 11.1/2.269 to get about 0.03 or so. I don't know how you obtained 1 for 11.1/142. Common sense should tell you that isn't 1. Good luck in the future. Chemistry is not hard but it is disciplined. Keep the formulas in your mind, punch in the right numbers in the calculator, always keep your common sense. The biggest problem I've seen in my career of teaching for 40 years is that students let the subject of chemistry intimidate them.I can ask this question. "If an apple weighs 1 lb what will 2 apples weigh" The immediate answer is 2 lb. BUT if I say 1 mol Na has a mass of 23 grams, what will be the mass of 5 mols Na? Many students will say, "I don't know because I'm not good at chemistry". They have let chemistry intimidate them. Anyway, good luck and you're always welcome to come to this site.
thanks
drbob222
Just follow the steps.
1. XHCOOH = mole fraction HCOOH.
2. XHCOOH = mols HCOOH/total mols.
3. mols HCOOH = grams HCOOH/molar mass HCOOOH
4. total mols = mols HCOOH + mols H2O. How much water is there? If you have 100 sample and it is 25.5% HCOOH, then 25.5 grams must be HCOOH and 100-25.5 = 74.5 g must be water. Then mols H2O must be grams/molar mass = 74.5/molar mass H2O = ?
5. Now you know mols HCOOH. You know mols H2O. Total mols = mols HCOOH + mols H2O and
6. Viola! XHCOOH = mols HCOOH/total mols.
The answer is approx 0.12 or so.
for which one?
molality or mole fraction?
mol fraction. I didn't do the molality. It's done the same say as the last one.
m is approx 7 if I didn't screw up.