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Topic: Calculations from back titration to determine empirical formula?  (Read 18115 times)

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Offline Malovane

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I have a lab experiment I must do to find the empirical formula of a compound. The procedure involves the following:

1.Weigh 0.5g calcium hydroxide into 50ml beaker
2.Pipet 15ml of 2.0M HCl into the beaker, dissolve the calcium hydroxide, then transfer contents into a 250ml volumetric flask, then dilute the flask to the mark.
3.Pipet three 20ml samples into erlenmeyer flasks, and add indicator drops
4.Titrate excess HCl in the three samples with standard NaOH solution (0.1M), and thus record the volume of NaOH used.

The prelab says the following:

The student will react a known mass of calcium hydroxide with a known (excess) amount of HCl. The excess HCl is then back titrated with a standard NaOH solution. From this information the student can calculate the moles of hydroxide in the calcium hydroxide as well as the mass of hydroxide in the calcium hydroxide sample. From the known mass of calcium hydroxide the student can thus calculate the mass of calcium in the original sample and thus the moles of calcium in the calcium hydroxide sample. Based on the moles of hydroxide and calcium in the original calcium hydroxide sample the student can thus establish the empirical formula of calcium hydroxide.

Keep in mind I simply can't point to a periodic table and say the empirical formula is Ca(OH)2. How do you do each step of these calculations to determine the empirical formula?

Offline Borek

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Offline skbuncks

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Re: Calculations from back titration to determine empirical formula?
« Reply #2 on: June 23, 2010, 08:41:48 AM »
Then write down what you know. Like how much HCl did you start with and how much NaOH did you use etc

skb

Offline halogen

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Re: Calculations from back titration to determine empirical formula?
« Reply #3 on: June 24, 2010, 01:34:55 AM »
Hey Malovane ...I think I'm in your class.  I too have the same lab to do tomorrow(you may have been Tuesday).  Your write up reads verbatum from my lab manual.  :)))  School starts with a K....teacher is JA??  Right? 

Here's what I've done for the pre-lab and I may be making an error - perhaps someone can point out what I'm doing wrong. 

First I identified the moles of HCl initially present which was 0.04

Then I figured out how many moles of NaOH reacted in the titration. 

NaOH and HCl are 1/1 so there was 0.00132 moles HCl in excess present. 

Which means that 0.04 moles HCl minus 0.00132 moles HCl leaves 0.03868 moles hcl that reacted with the ca(oh)2

I figured that the ca(oh)2 is 2/1 ration which gave me 0.07736 moles .

0.500g divided by 0.07736 moles = 93.28 which is NOT the molar mass of Ca(OH)2..?!!!!


Offline halogen

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Re: Calculations from back titration to determine empirical formula?
« Reply #4 on: June 24, 2010, 01:46:29 AM »
Help us...Malovane and I need assistance.

Offline halogen

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Re: Calculations from back titration to determine empirical formula?
« Reply #5 on: June 24, 2010, 01:47:41 AM »
can you not say help on here?  i wrote help and it showed up as delete me??

Offline Borek

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Re: Calculations from back titration to determine empirical formula?
« Reply #6 on: June 24, 2010, 02:43:26 AM »
First I identified the moles of HCl initially present which was 0.04

2.Pipet 15ml of 2.0M HCl into the beaker

Are you sure it is the same procedure?

Hard to tell anything not knowing numbers you put into your calculations, you listed only some intermediate results.
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Offline halogen

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Re: Calculations from back titration to determine empirical formula?
« Reply #7 on: June 24, 2010, 03:09:57 AM »
i'm working on the pre-lab question.

Which is: Assume that you have weighed out 0.5000g of calcium hydroxide.  Following exactly the same procedure as in this experiment, calculate the empirical formula of calcium hydroxide for the back titration of one 20.00mL sample which used 13.20mL of NaOH solution.  The molarity of the HCl solution is 2.000 M and the NaOH solution is 0.1000 M.

So here's what I've done - not sure where I'm going wrong. 

HCl  0.04 moles HCl originally present in 20.00mL sample of 2.000 M HCl

NaOH  0.00132 moles NaOH react with the excess HCl  (13.20mL of 0.1000M NaoH)

0.00132 moles NaOH reacts 1/1 with HCl so the NaOH reacted with 0.00132 moles HCl excess.

0.04 moles HCl - 0.00132 moles HCl = 0.03868 moles HCl reacted with the calcium hydroxide. 

Equation for HCl and Ca(OH)2 is:

2HCl + Ca(OH)2 = CaCl2 + 2 H20

but the H+ and the OH- are reacting on a 1/1...  ??  right??

so that means that there was 0.03868 moles hydroxide ions reacting. 

This means that...

0.03868 moles OH x 17g/mole = 0.65756g hydroxide in original sample but there's only 0.5000g in the original.

I'm making a major error and having a hard time...help??

Offline Borek

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Re: Calculations from back titration to determine empirical formula?
« Reply #8 on: June 24, 2010, 04:30:00 AM »
I think you are misreading the procedure. 20 mL is a sample pipetted from the 250 mL solution, it is not 20 mL of 2 M HCl.
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Offline halogen

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Re: Calculations from back titration to determine empirical formula?
« Reply #9 on: June 24, 2010, 10:48:41 AM »
Thank you!!!!  I was reading the pre-lab which is based on the lab(which I did not read). 

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