Any Chemistry Guru's out there? Molarity question..

[Spring]

Hey there! I figured I'd post a long shot for anyone who's familiar with molarity? Grade 11 Chemistry?

We just had our mole test on Friday, and one of the questions for molarity stumped me. I literally stared at the question for half an hour trying to figure itout. Looking back on it, I still can't figure it out because I was convinced there is a missing piece of information, but my teacher assured it is solvable.

So the question asked to find the molarity of liquid C[suB]6[/suB]H[suB]6[/suB] with a density of .8787 g/ml.Now I was stumped because the question does not give you a mass, so I just assumed 100g? Or would I have just used the density and assumed it was only a 1 L solution and changed .8787g/ml into .0008787g/L then divided it by the molar mass to get the moles of the solution and go from there?

Just a long shot in case anyone might know.. I'm sure I'll get the test back in a week or so, but it's been bothering me!

 

[aurora369]

Well, carbon has a molar mass of 12 g/mol and hydrogen has a molar mass of 1 g/mol, so C6H6 has a molar mass of 6*12+6*1= 78 g/mol.

So with a density of .8787 g/ml, molarity is 0.8787 g/ml * 1/78 mol/g * 1/1000 ml/L = 0.00001127 mol/L

I find the easiest way to work with this stuff is to use the units as a guide. You take what you have, and knowing what you want, you should be able to piece it together. Starting with the density, g/mL, you know you want to get rid of g, so it must be divided by the molar mass, and then you need to get mL into L.

Does that make any sense?

--Dawn

 

[Spring]

Yup, that makes sense!
I've been so used to getting the moles with having a mass to relate to, so was kind of stuck. Didn't realize until after the test that I could relate the density with the molar mass to get the molarity.

Thanks!

 

[aurora369]

No problem!

This molar stuff is kind of tricky, but once you get, you're good to go.

I use it all the time in school now a days, so it is almost second nature to me now. But I remember having to learn this stuff in grade 11, and I remember it being tricky to get the hang of. But now that I have it, I can pull it up even if I haven't used it for months.

--Dawn

 

[DeniseJP]

The only thing I remember is that Mole Day (in relation to chemistry, not the critters) is October 23rd...my son Benn took chem in 10th grade and still talks about "moles" to this day...

Denise

 

[XxMontanaxX]

I had a test on moles in my Chem class Friday...I failed it.