GC: What equation is being used?

[Jman1984]

In topscore test # 2:

What volume of HCL was added if 20 ml of 1 M NaOH is titrated with 1 M HCL to produce a pH = 2?
The answer is 20.4 ml

I don't understand their solution...what equation is being used. I get the whole idea of there being 40 ml of a .5 M solution but from there I'm lost. Please someone calrify. Taking the DAT Monday at noon.

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[Jman1984]

By the way...if I got a 16/18/18 respectively in the GC sections of Topscore and have reviewed Destroyer 4 times should I be Ok for actual DAT. I heard actual is much more straight forward? Plus upon review and seeing dumb mistakes I made...my averages changed to 19/20/20. Please give me some hope.

 

[dfymarine]

In topscore test # 2:

What volume of HCL was added if 20 ml of 1 M NaOH is titrated with 1 M HCL to produce a pH = 2?
The answer is 20.4 ml

I don't understand their solution...what equation is being used. I get the whole idea of there being 40 ml of a .5 M solution but from there I'm lost. Please someone calrify. Taking the DAT Monday at noon.

okay it is actually very easy when thinking qualitatively.

when you titrate 1M NaOH of 20 ml with 1 M of HCL, what do you see from this ? this is actually a neutralization rxn, in which the strong base reacts with the strong acid to produce salt and water.

so if you add the equal volume of HCL into the NaOH, you will get pH of 7 right? b/c they are all strong acid and base. so in order to get pH 2, you need to add more HCL into the solution. a pH of 2 would produce a concentration of 10e-2 M by using the log.

so by using titration equation to tirate the whole volume:

(20ml NaOH + 20ml HCL )*( 1 M ) = ( 0.1 M )*( # ml)

==> 0.4 ml

so additional of 0.4 ml into 20ml of HCL => 20.4 ml

if you know how to do destroyer, you will be fine!

 

[Sublimize23]

Like was said figure out what is in excess, obviously the H+ because it has a pH of 2. pH 2 = 1x10^-2M H+, so 0.01M.
(1MHCl)(Xml)=(0.01M HCl)(40ml + x) Distribute...
X=0.4+0.01x
0.99x=0.4ml, close enough, so 0.4 ADDITIONAL mL, so 20+ 0.4 = 20.4.

 

[suhshi]

so by using titration equation to tirate the whole volume:

(20ml NaOH + 20ml HCL )*( 1 M ) = ( 0.1 M )*( # ml)

==> 0.4 ml

so additional of 0.4 ml into 20ml of HCL => 20.4 ml

if you know how to do destroyer, you will be fine!

are your units right here?
and do you mean .01M?

im not seein how you get .4 ml

 

[Jman1984]

Another chemistry question:

Are melting point and boiling point postiviely correlated?
i.e. If one molecule has the highest boiling point will it also have the highest melting point?

 

[Jman1984]

Yet another question:
When labeling priorities of groups around a central carbon to determine R/S configuration...why does Br get priority over F?

 

[suhshi]

can you explain the how he got .4 ml

 

[r8 dds]

Yet another question:
When labeling priorities of groups around a central carbon to determine R/S configuration...why does Br get priority over F?

Because it's heavier.

 

[Jman1984]

Because it's heavier.

So this whole time I was wrong in labeling them based on electronegativity!? Or is F just a special instance? Or is priority always based on weight?

 

[suhshi]

its based on weight
not electronegativity

 

[Sublimize23]

are your units right here?
and do you mean .01M?

im not seein how you get .4 ml

His is supposed to say 0.01M, he is distributing the 0.01 to the (40+x)
coming from 40ml of solution you now have from a neutral solution. Also yes priority is based on atomic number (mass), memorize:
BCNOFNe
order of halogens: F, Cl, Br, I as you go down. This got me through organic, lol

 

[dfymarine]

are your units right here?
and do you mean .01M?

im not seein how you get .4 ml

oops, sorry guys

yah it is 0.01 M

and you need to convert the total ML into L before dividing the 0.01 M

==> 0.4 ml

hope it helps!

 

[Sublimize23]

Jman best of luck to you, I'll be there with you tomorrow, but my exam is at 9AM