Homework Help: science(chem)

Posted by ~christina~ on Sunday, December 2, 2007 at 4:11pm.

I did a dehydration rxn with 1.424g of 4-methylcyclohexanol with .40ml 85% H3PO4 and 6 drops of concentrated H2SO4.

I have to calculate the limiting reagent even though I do suspect it to be the 4-methylcyclohexanol.

I do know the ratio is 1mol H3PO4: 3mol 4-methylcyclohexanol.

However even though I have the weight of my 4-methylcyclohexanol I have the ml of my H3PO4 I added.

.40ml 85% H3PO4

I was wondering how do I calculate how many moles I have of it. Can I just use the density to convert it or not?
I was wondering that since it isn't full strength but 85% H3PO4. Would that affect the calculations?

Thank You

science(chem) - ~christina~, Sunday, December 2, 2007 at 4:35pm
I think I got it.

I looked it up online and found that concentrated H3PO4 is 85% and it is 15M so I'd have (.40ml)

which would = .006mol H3PO4 that I have.

Is this right?
science(chem) - DrBob222, Sunday, December 2, 2007 at 7:04pm
Close enough.
specific gravity 85% H3PO4 is 1.69.
Density = 1.69 g/mL.
mols H3PO4 = 1.69 g/mL x 0.4 mL x (85 g/100 g) x (1 mol/98 g) = 0.00586.

As you suspect, I would be VERY surprised if H3PO4 is the limiting reagent.
science(chem) - ~christina~, Sunday, December 2, 2007 at 7:30pm
Oh yep. I went on Wiki to verify the density from that site I looked up and I saw it was 1.69 which was close to 1.7 so I figured it was fine.

Thanks Dr.Bob

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