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Below are the 20 most recent journal entries recorded in Chemistry help's LiveJournal:

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Saturday, November 14th, 2009
7:05 pm
[oxfordmuse]
Need chem help?
Check out my journal- I've got help on how to do a problem with STP, where to find chemistry articles for chemistry summaries, and more help to come..
Wednesday, November 5th, 2008
6:02 pm
[tropicalstorm98]
equilibrium
I have a question about finding the equilibrium. My question given was to calculate the hydroxide ion concentration in your solution from the pH at equilibrium. My first question is how do I find the pH at the initial equilibrium condition. I did an experiment and added drops of NaOH in ten drop increments. 0 drops pH 1.16, 10 drops pH 1.20, 20 drops pH 1.25, 30 drops pH 1.30, 40 drops pH 1.37, 50 drops pH 1.43.
Thanks!
Monday, April 14th, 2008
11:15 am
[namedphoenix]
Nitrate precipitation
I am a chemistry teacher, and am stumped on a lab I am trying to develop. I'd like to create a lab that will test for nitrates without the use of store-bought indicators.

Ideally, and assuming it works, I would like to use a solution to test for nitrates. There are those fish tank and spa tester kits that test for nitrates, but I want the kids to really have to see the chemicals involved in the test.

Ideally, I'd like something that will form a precipitate when reacting with a nitrate solution, but any obvious reaction would be ok, too. Color change, for example. But in most of the reactions I'm trying, the nitrate is always not the compound forming the precipitate, instead being a spectator ion. I've got all sorts of nitrate compounds, and they could be used in a control situation, to show the precipitate formed from the nitrates. But! I cannot figure out how to get the nitrate to precipitate out.
Thursday, January 24th, 2008
6:06 pm
[akatsuki]
Buffers
A student dissolves 10.86 g of crystalline sodium acetate trihydrate (CH3COONa•3H2O) in a little water, adds 4.00 g of glacial (100%) acetic acid, and then distilled water to
make the total volume 500 mL. Calculate the molar concentration of the buffer and its pH.

My work so far
So for the trihydrate: moles = 0.092776mol
Acetic acid - 0.066608mol
volume = 500mL

But now I don't know what to do. :( Any pointers?
Wednesday, November 7th, 2007
5:46 pm
[matthew_yanni]
hey guys can u plz tell me what the activated complex of A2 + B2-->2AB
DOES any1 know the activated complex of Asubscript2 + Bsubscript2---->2AB

thnx id appreciate it
Tuesday, October 30th, 2007
7:16 pm
[ashleydior]
help in balancing equation?
CO2 + H2O --> C3H7OH

thank you so much for your help!
Wednesday, October 24th, 2007
7:17 pm
[maxcrummfan]
My version, versus, another version, interchangeable?
Hello All,
I'm in a college Chem 2 class and I've just recently taken a quiz.  The problem is, is that- I ulitmately got the correct answer, however, the approach of solving this problem is incorrect.  I was marked off points in the structure of solving this problem, however, my prof. didn't have time to go over the quiz with me and I want to be sure that my version, is theoretically and quantitavely, correct, and somehow my version can be interchangeable with the prof. version.  Any help, explanation, would be great.  Thank you for your time:

The Question:

A solution is prepared to be .20M HC2H3O2 and .4M KC2H3O2.  What is the PH of this solution? (Ka= 1.7 x 10^ -5 for acetic acid)

My version of solving this problem:

KC2H3O2 >>>>>  K  +   C2H3O2                                                                C2H3O2 + H20>>>>>HC2H3O2 +  OH
      0.4                    0.4          0.4                                                                          0.4            ----                  0.2               0

INTIAL:                           0.4          0.2              0
CHANGE:                      -x             +x              +x
EQUIL:                        0.4-x        0.2+x            x   


Kb=     Kw/Ka  =    1.00 x 10^ -14/ 1.7x 10^-5  =   5.88 x 10^ -10
Kb=    [HC2H3O2] [OH] /  [C2H3O2]

5.88 x 10^-10 = (0.2 + x) (x) /  (0.4 - x)

2.35 x 10 ^-10 = 0.2x

x= 1.18 x 10^ -9

-log(1.18x 10- -9) =   8.93  =  POH           PH= 14- 8.93 = 5.07


TEACHER VERSION:

HC2H3O2 + H2O >>>>>>  H3O + C2H3O2
     0.2             ----                      0            0.4


INTIAL:               0.2         0          0.4
CHANGE:           -x          +x          +x 
EQUIL:            0.2-x         x            0.4+x

Ka=    [H3O] [C2H3O2] / [HC2H3O2]

1.7 x 10^ -5 =   (x) (0.4 + x) / (0.2 -x)

8.5 x 10^-6 = x = [H]

-log(8.5 x 10^ -6) = 5.07 = PH 
Thursday, March 15th, 2007
11:11 pm
[laughtooloud]
Help.
Another take home quiz. My answers are behind the cut. Help me out, please. :]


<lj-cut text="Thanks in advance.">

Name the following compounds:
(NH4)2S: <b>Ammonium Sulfide</b>
N2O5: <b>Dinitrogen Pentoxide</b>
HNO2: <b> Nitrous Acid</b>
Mg3N2: <b>Magnesium Nitride</b>
Sn3P2: <b>Tin (II) Phosphide</b>
CF4: <b>Carbon Tetrafluoride</b>
PO4: <b>Phosphorus Tetroxide</b>
HI: <b>Hydroiodic Acid</b>
NH4OH: <b>Ammonium Hydroxide</b>
CaHCO3: <b>Calcium Bicarbonate</b>
 
Write formulas for the following compounds:
Iron (II) Carbonate: <b>Fe2(CO3)3</b>
Beryllium Phosphate: <b>Be3(PO4)2</b>
Boron Triiodide: <b>BI3</b>
Hydrobromic Acid: <b>HBr</b>
Carbon Disulfide: <b>CS2</b>
Acetic Acid: <b>HC2H3O2</b>
Aluminum Nitride: <b>AlN</b>
Leah (IV) Oxide: <b>Pb(OH)4</b>
Iron (II) Sulfide: <b>FeS</b>
Magnesium Hydrogen Sulfite: <b>Mg(HSO4)</b>

How many iodine atoms are in 1 unit of Ca(IO4)2?
<b>2</b>

How many total atoms are in 1 molecule of C6H12O6?
<b>180.16</b>

Calculate the molar mass of CH2O.
<b>34.02</b>

How many moles of oxygen atoms are in 1 mole of CO2 molcules?
<b>2</b>

How many moles of iodine atoms are in 2.5 moles of CI4 molecules?
<b>10</b>

How many moles of water are in a 25g sample of water?
<b>1.39</b>

What is the mass of a 3.8 mole sample of carbon dioxide?
<b>167.2</b>

Calculate the molar mass of C10H10 to 1 decmial place.
<b>130.2</b>


Current Mood: creative
Monday, March 12th, 2007
4:47 pm
[geek_dragon]
Rusty at Redox.
I was reducing vandyl sulphate (vanadium 4) to vanadium 3 in H2SO4 solution with a lead anode, and a plantinum cathode.  How do I figure out what form the V(III) is in (ligands, salt?); how do I figure out what the lead precipitate is, and why there was bubbling at the lead anode?  I'm going to cross post this because I'm confused.

EDIT:  I'm making vanadium (3) alum!  Somehow the alum is formed from the clay pot in electrolysis that reduces the vanadium from 4 to 3 in sulphuric acid.  What is the M+ in the alum formula?  (alums are (M+)2(SO4)*(M+3)(SO4)3* 24 H20, vanadium is the M+3)
Sunday, March 4th, 2007
6:11 pm
[trebizond]
ok, hi guys, i'm erin and i have just spent like 5 hours trying to figure out stupid titrations. grr, and im not exaggerating ther.
i found this one that is similar in the textbook, so if someone could show me how to do this, it would be GREATLY appreciated. 

To analyse a sample of vinegar for its acetic acid content, a student began by accurately diluting a 20.00 mL sample of vinegar to a 250mL volumetric falsk. 
 20.00mL samples of this diluted vinegar were then placed in conical flasks and titrated against a 0.0500M solution of sodium hydroxide. the average titre obtained was 21.52mL. 
 Assuming that acetic acid is the only acid present in the vinegar, calculate the concentration of the acetic acid in the vinegar (in g per L)

i keep getting 3 kilos, and im pretty sure that that is not right. lol. 
Tuesday, February 6th, 2007
6:33 pm
[laughtooloud]
Hello.
Luckily my Chem teacher doesn't assign homework problems from the book but I was reading through them and one of them caught my eye. The question is..

The slogan "When it rains, it pours" has been associated with a brand of table salt for decades. How can salt, a solid, be poured? What unique feature at one time, but not today do you suppose was being emphasized by the slogan? In other words, under what circumstances would one brand of salt 'pour' while another brand would not and why?

I understand that a salt can be poured because they're small soild chunks but the last part stumps me. Any ideas?

Current Mood: Wondering.
Wednesday, January 31st, 2007
12:38 am
[laughtooloud]
Just a few.
Here's some good ol' Chemistry practice problems that I would like someone to double check for me, please. :)

Blah blah blah.Collapse )

Current Mood: Confused.
Sunday, January 28th, 2007
3:07 pm
[depphead]
AP Chem
I'm working on a take home test for Chemistry and there are a few questions I'm a little unsure about, if someone could help explain any of them to me I would really appreciate it...

1.) Which of the following would you expect to have the highest boiling point?
[A] Cl2
[B] I2
[C] F2
[D] Br2
[E] All of these have the same boiling point.

2.) Which of the following is the correct order of boiling points for KNO3, CH3OH, C2H6, Ne from lowest to highest?
[A] Ne, C2H6, CH3OH, KNO3
[B] Ne, C2H6, KNO3, CH3OH
[C] C2H6, Ne, CH3OH, KNO3
[D] KNO3, CH3OH, C2H6, Ne
[E] Ne, CH3OH, C2H6, KNO3

3.) The molecules in a sample of solid SO2 are attracted to each other by a combination of:
[A] London forces and H-bonding
[B] H-bonding and ionic bonding
[C] London forces and dipole-dipole interactions
[D] Covalent bonding and dipole-dipole interactions
[E] none of these

4.) Knowing that ΔHvap for water is 40.7 kJ/mol, calculate Pvap of water at 37°C.
[A] 25.4 torr
[B] 52.6 torr
[C] 12.4 torr
[D] 18.7 torr
[E] 6.90 torr


5.) The molar volume of a certain form of solid lead is 18cm3/mol. Assuming cubic closest packed structure, determine the radius of a Pb atom:
[A] 1740 pm
[B] 174 pm
[C] 17.4 pm
[D] 1.74 pm
[E] none of these

6.) When a water molecule forms a hydrogen bond with another water molecule, which atoms are involved in the interaction?

7.) Sodium Oxide (Na2O) crystallizes in a structure in which the O2- ions are in a face-centered cubic lattice and the Na+ ions are in tetrahedral holes. The number of Na+ ions in the unit cell is:
[A] 8
[B] 4
[C] 2
[D] 6
[E] none of these

8.) Aluminum metal crystallizes in a face-centered cubic structure. The relationship between the radius of an Al atom (r) and the length of an edge of the unit cell (E) is:
[A] r = E/2
[B] r = 2E
[C] r = (√3/4)E
[D] r = 4E
[E] r = (√2/4)E

9.) The vapor pressure of water at 25°C is 23.8 torr; ΔHvapor = 43.9 kJ/mol.
a. Determine the vapor pressure of water at 70°C.
b. Determine the percentage error if your experimental results for vapor pressure at 70°C give a result of 225 torr.
c. Give structural reasons that support the fact that the boiling temperature of C2H3OH is 78°C and water is 100°C (both at 760 torr).

NOTE: I'm not just posting my whole test on here, I just numbered them so the post is easier to read.

Any help is appreciated.
Wednesday, January 24th, 2007
11:33 pm
[thomnkyansgirl]
Hi! I know we're not supposed to say this, but I missed a week of class (2 classes) due to being hospitalized, and I'm having a really hard time figuring out conjugate acids and bases. This is the problem:

Write in the boxes formulas for the conjugate acids and conjugate bases of the following substances. If there is none, write 'NONE'.

HPO42-

HCO3-

H2O (which I know can be either an acid or a base)

Cl-

NH4+

I don't know very much about chemistry, but I really want to learn how to work these. Any help you could give will be greatly appreciated.
9:51 pm
[laughtooloud]
HELP! :)
Hey, I'm new here and have a take home quiz and I just want to get some double checks on the answers I have. I'd love for you guys to tell me what you'd get for the answers. Thanks!

Chemistry.Collapse )

Current Mood: Upset.
Monday, December 11th, 2006
12:16 am
[tropicalstorm98]
Assignment # 3
Once agin I want to thank everyone here for their help on my last assignment. I got an 80. Now I need help for this very last assignment.


*Question 1
What is the maximum number of electrons that can fit into the 5f subshell?

6
14
8
10
18
5

I want to say 18 but i'm not sure.

*Question 2

What is the correct chemical formula (not name) for the ionic compound that forms between potassium and oxygen?

Is the answer KO?


*Question 3
What is the charge on the tin ion in the compound below?

SnO

-1
+3
-2
+1
-3
+2

Metal tend to lose electron correct? if so the answer is foing to be a positive charge, but I don't know which one.

*Question 4
What is the correct name of the compound below?

CuCO3

copper monocarbonate
copper(I) carbonate
copper carbonate
copper(II) carbonate
copper carbontrioxide


I'm really bad at these but, I'm leaning towards the first one cuz of the "mono"


*Question 5
Classify the reaction below.

Ti + 2Cl2 → TiCl4

combination
decomposition
combustion
single replacement
double replacement

I think Its a single replacement, because Ti didn't gain or lose anything. But...hmm...combination. i'm not sure about that one.

*Question 6
Use the reaction below to answer the following true/false question

Zn(s) + Fe2+(aq) → Fe(s) + Zn2+(aq)


The iron(II) ion is the reducing agent in this reaction

True/false?

true because iron gave electrons to copper

*Question 7
Aqueous solutions of which two reactants below will produce a precipitate when mixed?

CuSO4 and Pb(NO3)2
NH4Cl and BaCl2
K3PO4 and NaCl

I need some help with this one. I could figure it out possibly if i knew the charges of each one.


*Question 8
You must express your answer to the correct number of significant figures to receive any credit. Enter only the numerical answer.
Use the balanced equation below for the following calculation.
4Fe + 3O2 → 2Fe2O3

_____ moles of O2 are needed to react completely with 9.00 moles of Fe.

I want to say 48 because, before the reaction occurs Oxygen is, 3O2. and one mol of O is 16 so 6*16= 96. when the reaction occur, Oygen go from 3O2 to O3. and 16*3= 48. is this right or am I totally off. Please be blunt if you must.


*Question 9

Use the balanced thermochemical equation below to answer the following problem. Your answer must contain the correct number of significant figures and be rounded off correctly to receive any credit. Enter only the numerical answer.
N2 + 3H2 → 2NH3 + 22.0 kcal
The reaction of 155 grams of N2 produces ________ kcal of heat.

I am clueless on this one. i think I am supposed to divide 155 by 22 but i'm not sure. Help?

*Question 10

Your answer must contain the correct number of significant figures and be rounded off correctly to receive any credit. Enter only the numerical answer.
A sample of gas has a volume of 5.2 L at STP. This sample of gas will occupy _____ L when the pressure changes to 962 mmHg while the temperature remains constant.



If someone can tell me what gas law i should use: ( Boyle, Charles, Gay-Lussac ,etc) I can figure this out

Thank you all for the help.
Any is appreciated
Sunday, December 10th, 2006
7:04 pm
[depphead]
Sorry about two posts in one day. I'm working on a take-home quiz and I'm still stuck on the following problems:

7.) What is the wavelength of light that is emitted when an excited electron in the hydrogen atom falls from n = 5 to n = 2?
[A] 5.82 x 10-7m
[B] 4.34 x 10-7m
[C] 5.12 x 10-7m
[D] 6.50 x 10-7m
[E] none of these

11.) Consider the following portion of the energy-level diagram for hydrogen:
n=4 -0.1361 x 10-18 J
n=3 -0.2420 x 10-18 J
n=2 -0.5445 x 10-18 J
n=1 -2.178 x 10-18 J
For which of the following transitions does the light emitted have the longest wavelength?
[A] n = 4 to n = 3
[B] n = 4 to n = 1
[C] n = 4 to n = 2
[D] n = 2 to n = 1
[E] n = 3 to n = 2

13.) Consider the following portion of the energy-level diagram for hydrogen:
n=4 -0.1361 x 10-18 J
n=3 -0.2420 x 10-18 J
n=2 -0.5445 x 10-18 J
n=1 -2.178 x 10-18 J
In the hydrogen spectrum, what is the wavelength of light associated with the n = 2 to n = 1 electron transition?
[A] 1.097nm
[B] 364.9nm
[C] 9.122 x 10-8m
[D] 0.1097 x 10-8cm
[E] 1.216 x 10-7m

17.) Which of the following combinations of quantum numbers do not represent permissible solutions of the Schrodinger equation for the electron in the hydrogen atom (i.e., which combination of quantum numbers is not allowed)?

[A] Option B
[B] Option D
[C] Option A
[D] Option C
[E] All are allowed.

28.) For the set of elements Li, O, Ne, and Na, which element has the largest atomic radius? Explain and deviation from the expected pattern.

30.) Photogray lenses incorporate small amounts of silver chloride in the glass of the lens. The following reaction occurs in the light, causing the lenses to darken:
AgCl(s) --> Ag (s) + Cl
The enthalpy change for this reaction is 3.10 x 102kJ/mol. Assuming all this energy is supplied by light, what is the maximum wavelength of light that can cause this reaction?


Any help is greatly appreciated.
4:38 pm
[depphead]
Which of the following statements is (are) true?
I. An excited atom can return to its ground state by absorbing electromagnetic radiation.
II. The energy of an atom is increased when electromagnetic radiation is emitted from it.
III. Th energy of electromagnetic radiation increases as its frequency increases.
IV. An electron in the n = 4 state in the heydrogen atom can go to the n = 2 state by emitting electromagnetic radiation at the appropriate frequency.
V. The frequency and wavelength of electromagnetic radiation are inversely proportional to each other.

[A] III, IV, V
[B] II, III, IV
[C] III, V
[D] I, II, III
[E] I, II, IV


Can someone tell me what the answer is, and explain why each is right or wrong?
Monday, December 4th, 2006
7:08 pm
[tropicalstorm98]
I want say thank you for the help i receive last time i was here. I truly appreciated it. I have some more questions that I need help with.

Questions

Q1: What is the osmolarity of a 0.20 M solution of MgSO4 ?

a.)0.40 osmol
b.)0.20 osmol
c.)0.050 osmol
d)0.80 osmol
e)0.10 osmol


I'm Leaning towards a and d for this question. to get the osmolarity you multiply the molarity M by the number of particles. And I wanted to choose a because there is a magnesium particle and a SO4 particle and the answer would be: (0.20 x 2= 0.40), but am I suppose to count that SO4 as 4 separate particles? Thus making it be :(0.20 x 4= .80). Which would it be?


Q2: What is the formula for the salt that is produced when an aqueous solution of LiOH reacts with an aqueous solution of HI ?

a.)LiH
b.)LiI
c.)Li2O
d.)IOH

I was leaning toward because I figure that the hydrogen from the "I" would have bonded with the OH to make H2O leaving Li and I to bond and produce LiI. That's the only thin that makes sense to me. is that correct?


Q3: Which two substances in the equilibrium below are reacting as Bronsted-Lowry bases?

H2SO4 + CN ‾ ↔ HSO4‾ + HCN

a.)H2SO4
b.)HCN
c.)HSO4‾
d.)CN ‾

I'm pretty sure it's CN‾ and HSO4‾ because when you look at CN‾ then HCN in the equilibrium you see that CN‾ accepted the H making it HCN. And because it accecpt H, it's an acid and that means CN‾ had to have been a base. and the only base there could be left is the other base next to the other acid, that is the base HSO4‾.


Q4: What are the conjugate acid-base pairs in the equilibrium below? (You must select two choices from the list of possible answers).

a.)F ‾ and HF
b.)HF and NH3
c.)F ‾ and NH4+
d.)NH4+ and NH3
e.)F ‾ and NH3
f.)NH4+ and HF

F ‾ + NH4+ ↔ HF + NH3

I'm gonna say answer a and d (for the same reason in question 3)


Q5: What is the pH of a solution where [OH‾] = 1.2 x 10^-5 M ?

a.)12.00
b.)1.20
c.)8.33
d.)4.92
e.)9.08

okay, to get the pH you take the -log[H3O+]. Now am I supposed to take the -log of 1.2 x 10^-5 M? When I did that in my calculator I got 3.9208187 which I rounded to 3.92. Because of this I thought i did it wrong, but then i did another pH question to help me figure out this one. It was (1.4 x 10^-9)- and I got 7.85. Well the answer in the book was 8.85. So one each my answer was one whole number down from the answer in the book and the assignment. So do you think it's 4.92? answer d?
Sunday, November 26th, 2006
7:05 pm
[illicit_]
Stoichiometry.
I have a couple of questions here.

1: CAN YOU HELP ME BALANCE THIS---->NH4OH+CuN2O3+Cu02H2

2: How do you do Limiting and Excess Problems when your givens are in Moles?

3: When 50.00 grams of STEAM (I know what steam is but what is the molecular equation?) is passes over 100.0 grams of Iron, Hydrogen gas and iron(III)oxide are formed.

This is really frustrating me so much! My teacher never taught me this stuff, SO I am depending on you guys to help me.
THANKYOU!
=D
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