Need help as to going about a Chem problem

blue powder

Guys, I was looking for some direction with this chem problem. I converted molarity of the dichromate to moles, then i used the stoichiometry ratios to convert moles of C2H5OH, then that to grams. But I realised I already have the grams of that so I've just lost myself in a page of equations!

Essentially I don't have a problem with calculating, I just don't see what this questions wants me to do. What on earth is the plasma? Is it the C2H5OH? Then is this the "mass percent" they're looking for?

Thanks in advance for the direction!

Hahnsoo1

You don't already have the grams of C2H5OH, you have the grams of plasma (which is a whole mess of things, including blood alcohol). Just divide your answer by 28.50g of plasma (since the grams of C2H5OH is a raw answer and not a percent) to come up with a percent, then convert the percent using the parts per 100 parts of blood (in this case, parts = grams). According to the question, your answer will be g alcohol per 100 g of blood, I guess.

blue powder

Thanks, Hahnsoo, I'm not sure I completely follow you here, but the answer I ended up with is 0.1585g alcohol per 100g blood. I got this answer by dividing grams of blood by grams of C2H5OH and multiplied tha by 100. I'm dubious of this answer.. I'm having a really hard time with this question!

pengu

Hello hello
Based on the balanced equation you can tell that you need two molar equivalents of Cr2O7 per mole of ethanol (C2H5OH). Then, from the second half, figure out the number of moles of Cr2O7 required which would also tell you the number of ethanol present in the plasma sample. At which point simply do a ratio of EtOH to blood where the blood is 100g instead of the 28.5g. (cross multiply) Voila!

blue powder

Okay, I divided grams of ethanol hydroxide by grams of plasma and got 0.1807/28.50, so then do i cross multiply this by 28.50/100? I can't see in my head how i'm goign to convert this into per 100g.

Right now I have the asnwer 0.02225g of C2H5OH/100g of plasma.. I think this looks right, but I want to understand what I've just done!

pengu

Ok, so

From the number of moles of Cr2O7 you figured out the amount of ethanol in that 25.80g of plasma/blood yes? Convert that mole to a mass ( multiply by molar mass)

grams of ethanol (you just figured out this one) x grams of ethanol

---

=

---

28.50g plasma 100g plasma

now cross multiply to figure out x which is in grams. Divide that over 100g to find a percentage.

blue powder

I got it! 0.03509% C2H5OH! Thank you all very much for your help, I really appreciate it. Looks like I'm going to have to really work on this sort of thing.

I assume you guys have been through a lot of chem or atleast did it at college, any advice for someone studying a science degree?

corcorigan

Don't panic.

Just work out what you need to do before you jump in there and confuse yourself with a page of irrelevant numbers. That and practice.

Actually that's probably quite a good bit of advice for most things, and I should heed it more often. Sigh.

Fuzzy Cumulonimbus Cloud

blue powder wrote: »

I got it! 0.03509% C2H5OH! Thank you all very much for your help, I really appreciate it. Looks like I'm going to have to really work on this sort of thing.

I assume you guys have been through a lot of chem or atleast did it at college, any advice for someone studying a science degree?

The first thing I tell anyone who is doing lengthy chem problems is to immediately write on your paper a ?bloodalcohollevel, or ?molar if it was a molar problem, or ?joules if it was an energy problem. Then circle what you know to get there. Then start your dimensional analysis. Ignore the distractions. Also, a titration problem should tip you off that there is going to be some mole to mole ratios, because that's how titrations work (you use one part of a known equation to figure out the concentration of another part of the equation in solution).


And joke note: C2H5OH is ethanol, not ethanol hydroxide. The OH makes it an alcohol. Stick with chem and you will learn all about alcohols, and carboxylics, and...organic to your hearts desire.

blue powder

Ohh haha, thanks a lot for that little Ethanol tip! Yeah it is all very intimidating and daunting but I am enjoying it. I'll be doing carbon chem next sem, looking forward to it! Thanks, guys, you've all been excellent!