posted
I just started a Chemistry course and they gave us two "pre-lab" questions for our first lab, but I can't remember how to solves these kind of problems or find any info on how to do so... All we've done in class so far is review how to do dot diagrams and molecular geometry so that was not helpful for this problem either. Any help would be appreciated! The only other Chem course I took was about 3-4 years ago and I can't for the life of me remember how to answer this!
Any links that would explain how to calculate the answer or any explanations or descriptions on how you solve this type of problem would be helpful.
Problems: 1. How many millileters of 0.25 M NaOH should be needed to titrate a 0.5000 g sample of KHP?
2. How many milliliters of 0.25 M NaOH should be needed to titrate a 5.00 mL sample of vinegar that is 4.0% by mass acetic acid and has a density of 1.0 g/mL?
Any links to explanations of how to answer these type of problems would be very appreciated, I did try a web search but was not able to find anything helpful.
(ETA mL)
-------------------- Before you criticize someone, you should walk a mile in their shoes. That way, when you criticize them, you're a mile away and you have their shoes. Posts: 2110 | From: Chicago, IL | Registered: Jul 2000
| IP: Logged |
posted
These are simple acid base reactions (though it has been years since I had chem.)
Check your lab book or text book on titrations and acid/base reactions. This may bring back some memories. You are going to have to figure out how much acid you have to figure out how much base (NaOH) you need.
Also on number 2, is is 5 mL of vinegar?
Posts: 20 | From: Oklahoma | Registered: Aug 2005
| IP: Logged |
posted
The M means moles per liter. Moles are a measure of numbers of particles. So first calculate how moles of KHP it takes to make .5g. Then figure out how many particles of NaOH it takes to neutralize how many particles of KHP. In this case the ratio is 1:1. Then you know how many moles of NaOH you need. Divide that by the molarity to get the number of liters you need, then multiply by 1000 milliliters per liter.
The second is the same except you have to do an extra bit of a calculation first to figure out how many grams of acetic acid you actually have.
Posts: 2018 | From: Santa Barbara, California | Registered: Aug 2005
| IP: Logged |
posted
I suspect that your answer should also include the correct number of significant figures. You were given starting numbers with different numbers of significant figures, probably to see if you know how to determine the correct number.
A bit of general advice: chemistry labs often result in complete failure because of crappy equipment. Always do a complete write-up of what should have happened as well as why you suspect it didn't. You can usually get almost full credit for no results if you can explain everything.
-------------------- "Science makes everything sound painful, Spongebob." - Sandy Posts: 225 | From: Salt Lake County, UT | Registered: Sep 2005
| IP: Logged |
quote:Originally posted by Kahdra: I suspect that your answer should also include the correct number of significant figures. You were given starting numbers with different numbers of significant figures, probably to see if you know how to determine the correct number.
How embarrasing! My chem teacher went on about that constantly; he'd be disappointed
Ok, so possible answer is 9,793 ml. then.
And I absolutely agree on the second point: always explain possible...I don't know the English word, it's "failure sources" in Norwegian. Anyway, explain what might have gone wrong and distorted or contaminated the results.
posted
5 ml of 4.0% acetic acid at a density of 1 g/ml = 5g x 0.04 = 0.2 g of acetic acid (CH3COOH)
mw of CH3COOH = 60.05 grams
so number of moles of 0.2 g of acetic acid = 0.2 g/60.05 g/mole = 3.33 x 10-3 moles of acetic acid
V (NaOH) = n/c = (3.33*10-3/0.25) * 1000 ml
= 13.3 ml
Double check the math.
-------------------- "The women who embraced in the wagon were Adam and Eve crossing a dark cathedral stage—no, Eve and Eve, loving one another as they would not be able to once they ate of the fruit and knew themselves as they truly were." - Lynn Cheney, Sisters. Posts: 289 | From: California | Registered: Apr 2005
| IP: Logged |
posted
Oops, browser glitch. Wish they had the internet when I took Chemistry and I could have someone else do my homework.
-------------------- "The women who embraced in the wagon were Adam and Eve crossing a dark cathedral stage—no, Eve and Eve, loving one another as they would not be able to once they ate of the fruit and knew themselves as they truly were." - Lynn Cheney, Sisters. Posts: 289 | From: California | Registered: Apr 2005
| IP: Logged |
posted
I am pretty sure you go with the lowest number of sig figs in a multiplication/division problem, so the correct answer would have two digits (from the 0.25 M NaOH in the first problem and the 0.25, 1.0, etc. in the second). I don't know where any of my old chemistry textbooks are to confirm this, though.
-------------------- "Science makes everything sound painful, Spongebob." - Sandy Posts: 225 | From: Salt Lake County, UT | Registered: Sep 2005
| IP: Logged |
posted
I think they are ambiguous problems. What does "titrate" mean in this case? To get to a neutral pH? To add the same number of moles? Those are rarely the same thing. Usually it means "to get to a desired state," so one would usually want to titrate something to a certain pH, but that's not specified here.
Also, "KHP" is not a formula, but an abbreviation for something. I presume that potassium hydrogen phosphate is meant, but that could be either K2HPO4or KH2PO4.
If you just want to know how many moles of NaOH are equal to the number of moles of "KHP," then others have given you some tips (but it won't likely be a neutral pH when you are done, certainly not if you are using K2HPO4).
If this is a more advanced chemistry class, then you might have to specifiy a pH to titrate to, and you will have to use the Henderson-Hasselbalch equation (not be confused with the Anderson-Hasselhoff equation, which is used on the beach), and you will also need to know the acid dissociation constant (pKa) for the substance you are titrating (if it's phosphate, there are three, but the one near neutral is 7.21).
UBBFriend: Email this page to someone!
Printer-friendly view of this topic
![[dunce]](graemlins/dunce.gif)
![[Big Grin]](biggrin.gif)
), but in general, what you need to know is how to go via mol:![[Razz]](tongue.gif)
![[Wink]](wink.gif)
![[Embarrassed]](redface.gif)
How embarrasing! My chem teacher went on about that constantly; he'd be disappointed 
