Friday 24 August 2012


Chem! HELP!! :) :) :)?

Vanadium crystallizes in a body-centered cubic lattice (the V atoms occupy only the lattice points). How many V atoms are present in a unit cell?!

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Body-centred cubic lattice means the eight corners of a cube and the centre of the cube are occupied by vanadium atoms.

Each corner is shared by eight neighbouring cells. Hence, each corner contributes 1/8 atom.

So, there are
8 x 1/8 = 1 atom

The atom in the centre of the cube is totally belongs to the unit cell. So, it contributes one atom.

Overall, the number of V atoms in a unit cell is 1 + 1 = 2

Chemistry question - using the integrated 1st order rate law...please help!?

Consider two reaction vessels, one containing A and the other containing B. At t = 0, [A]o = [B]o. A and B decompose by first order kinetics with rate constants of
kA = 4.50 × 10-4 s-1 and kB = 3.70 × 10-3 s-1, respectively. Calculate the time that must pass to reach the condition such that [A] = (4.18)[B].

Please work out the problem and explain to me how you find the concentrations of A and B to start with. Thank you!

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For A: ln [A]o/[A] = k' t
ln[A]o - ln[A] = k' t
ln[A]o = k' t + ln[A]....................(1)

For B:
ln [B]o = k" t + ln[B]

Since [A]o = [B]o

ln [A]o = k" t + ln [B] ...............(2)

(2) = (1)

k" t + ln [B] = k' t + ln [A]

Given that [A] = (4.18) [B]

(k" - k' ) t = ln (4.18)[B] - ln [B]

(k" - k') t = ln 4.18 .......................{ln X - ln Y = ln X/Y}

t = ln (4.18) / (3.70 x 10^-3 - 4.5 x 10^-4)

t= 440.1 s

Wednesday 22 August 2012


How does electronegativity relate to acidity in organic compounds?

List the following carboxylic acids in order of decreasing acidity:

CH3CHClCOOH, CH3CH2COOH, CH2BrCH2COOH, CH3CCl2COOH

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CH3CCl2COOH > CH3CHClCOOH > CH2BrCH2COOH > CH3CH2COOH

A very electronegative atom will attract the electrons towards itself. This will reduce the electron charge density of the OH bond in COOH group. It weakens the OH bond and it is able to donate the proton (H+) more easily and hence a stronger acid.

[Acid is a proton donor]

The electron withdrawing effect is stronger in Cl than in Br because Cl is more electronegative than Br.

The withdrawing effect is stronger if the electronegative atom is nearer to the COOH group.

Explain why a strong acid and weak base produce acidic salt?


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NH4Cl is an acidic salt because it is formed from the reaction between HCl (strong acid) and NH3 (weak base).

NH4Cl is acidic salt because when it dissolves is water, it undergoes cathionic hydrolysis. The NH4+ ions will react with the water molecules to form NH3 and H3O+.

NH4+ + H2O <------> NH3 + H3O+

The formation of H3O+ causes the solution to be acidic.

Cl- will not react with water molecules to form HCl and OH- because HCl is a strong acid and will dissociate completely.

Anyone, answer this Dalton's Law of Partial Pressure?

A mixture of gases contains 97% by volume CH4, 1.5% C2H6 by volume, 1% Co2 by volume and 0.5% N2 by volume. This was collected at a total pressure of 101KPa. What is the partial pressure(in atm) of each gas in the mixture?

Thank you so much.

Best Answer - Chosen by Voters

Since the volume of gas is proportional to the number of moles, we can directly use the percentage of volume of gas to calculate the partial pressure of each gas.

Using the Dalton's Law:

Partial pressure = mole fraction x total pressure
........................= % volume x total pressure

Hence, P(CH4) = 0.97 x 101 = 97.97 kPa
P(C2H6) = 0.015 x 101 = 1.515 kPa
P(CO2) = 0.01 x 101 = 1.01 kPa
P(N2) = 0.005 x 101 = 0.505 kPa

The unit of pressure can be converted by:

101325 Pa = 1 atm

Friday 17 August 2012


Help! stuck on this questions about Germanium, Ge.?

1)the table below gives the relative abundance of each isotope in a mass spectrum of a sample of germanium , Ge.

m/z: 70 72 74
Reletice abundance (%): 24.4 32.4 43.2

i) Copy and complete the electron arrangment of a Ge atom.
1s^2

ii)Use the data above to calculate the relative atomic mass of this sample of Germanium. give answer to 1.d.p

iii) State what is adjusted in a mass spectrometer in order to direct ions with different m/z values onto the detector. Explain your answer.

thanks for the help guys x

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1) Ge has 32 electrons.
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p2

2) RAM = { 70(24.4) + 72(32.4) + 74(43.2) } / 100 = 72.4

3) The strength of the magnetic field is adjusted in order to direct ions onto the detector.

The angle of deflection of ion is proportional to the charge and is inversely proportional to the mass of ion.

This means ion with small value of m/z (Ge-70)+ is easily deflected and is directed to the detector when the strength of the magnetic field is low. As the strength of the magnetic field is increased, the ion with higher m/z value will be directed on to the detector.

Which tests help to distinguish between methanol and ethanol?


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Ethanol will give a positive test with the iodoform reagent.

Add a solution of iodine in alkaline condition (NaOH) to ethanol. Heat it and a yellow precipitate of iodoform will form.

methanol has no reaction with iodoform reagent.

Conditions of positive iodoform test:
The carbon atom must attach to i) CH3....ii) OH.......iii) H

Rate Constant Chemistry?

What is the rate constant for a second order reaction when the initial [A] = 0.20M, the initial [B] = 0.86M and the initial rate = 0.0900 M/s?

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For the second order reaction, the rate equation is

rate = k [A] [B]
substitute the values given,

0.09 = k( 0.2) ( 0.86 )

solve for k.

What is the relationship between structure and Rf value?


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Rf stands for the retention factor in chromatography. It is a measure of how strongly the substituents are attached (adsorp) to the stationary phase. The stronger the adsorption, the lower is the Rf value.

Rf can be calculated by dividing the distance covered by the substituent with distance covered by the mobile phase.

A bulky structure ( branched isomer) has a weaker bond compared to a straight chained isomer. This is because the area of contact betweeen molecules is less compared to the straight chained isomer. ( van der Waal forces) Hence it moves faster and have a higher Rf value.

A molecule that can form hydrogen bond will have a lower Rf value compared to molecule with van der Waals as its intermolecular force of attraction. 

How do I determine which molecules are polar?

I am given CCl4, CH4, H3C-CH2-CH2-CH2-CH2- CH3, H2O, CO2, and O2

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To determine the polarity of a molecule, you need to know two things.

a) Determine whether the bonds are polar?
b) Determine whether the molecule is symmetrical.(shapes)

A polar molecule must have polar bond and the molecule must be assymmetrical (not symmetrical).

A polar bond is formed if the atoms are of different electronegativities.

a) CCl4 -----> polar bond but symmetrical---> non polar
b) CH4 -----> polar bond but symmetrical---> non polar
c) same as above
d) H2O -----> polar bond and assymmetrical (V-shaped)--> polar molecule
e) CO2 -----> polar bond but linear in shape ----> non polar
f) O2 --------> non polar bond-----------------------> non polar

What is the rate law of the following mechanism:?

E + S <- -> ES (fast
ES -> E + P (slow)

E is enzyme, S is substrate, P is product.

Help?

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let the rate constant of the forward reaction be k1;
let the rate constant of the backward reaction be k2;
let the rate constant of the forward reaction of the second eq be k3

For the first reversible equation:(fast)
forward reaction: rate = k1 [E][S]
backward reaction: rate = k2 [ES]

At equilibrium, rate of forward reaction = rate of backward reaction
k1[E][S] =k2[ES]

[ES] = { k1[E][S] } / k2 ......................(1)

For the slow reaction:

rate = k3 [ES]...................................(2)
Substitute (1) into (2)

rate = (k3) { k1[E][S] } / k2

rate = k [E] [S] ; where k = (k3)(k1) / k2

Will C2H5F or C2H5OH have the higher boiling point? why?

This is a question about Chemistry. London Forces, Hydrogen Bonding and Dipole-Dipole.

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C2H5OH would have a higher boiling point because it can form hydrogen bonds between its molecules. So, more energy is needed to break the strong hydrogen bond.

In order to form hydrogen bond, the hydrogen atom must directly attached to a very electronegative atom such as N, O or F.
In C2H5F, all the H are attached to the carbon atom whereas in C2H5OH, one of the hydrogen atom is attached to the oxygen atom.

Further more, the size of C2H5OH is bigger than C2H5F. The bigger the size, the stronger is the London Force.

Chemistry A level equilibria problem?

just a bit confused how to do this question....

On heating, X decomposes reversibly according to the equation:

X (g) ↔ Y (g) + Z (g)

This reaction was allowed to reach equilibrium at 2 different temperatures.

(a) When 1.00 moles of X was heated at 200'C in a container of volume 25.0dm3, the equilibrium mixture was found to contain 0.200 mole of Y. Calculate the value of Kc at this temperature.

if anyone can help, much appreciated :)

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........................ .X (g) ↔ Y (g) + Z (g)

initially ..............1.00.......0..........0
change...............0.2........0.2....…0.2

At equilibrium .....0.8........0.2.......0.2

concentration.......0.8/25 0.2/25 0.2/25

Kc = [Y] [Z] / [X]

......= {(0.2/25) (0.2/25)} / (0.8 /25)
......= 2.0 x 10^-3

Thursday 16 August 2012


Why is the oxide of si solid while the oxide of c is gas at room temperature?


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The oxide of carbon, CO2 and CO exist as simple melecular structure. The intermolecular forces of attraction between the molecules is the weak van der Waals force. Hence they have low melting and boiling points and at room temperature and atmospheric pressure , they exist as gas molecules.

The oxide of Si, that is SiO2 exist as giant covalent structure. The Si atom and oxygen are bonded together by strong covalent bond tetrahedrally forming a very big molecules. A large amount of energy is needed to break the covalent bonds as well as the giant structure.

That's why it has a very high melting and boiling points. Hence, it exists as solid at room temperature.

PH chemistry question help!?

Could someone help with this question? And explain why

A known volume )1mL) of a 0.100M NaOH solution was diluted to a total volume of 10.0mL (Solution A). Next 1.00mL of this solution was diluted to a total volume of 50.mL (Solution B) What is the pH of solution B?

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Using M1V1 = M2V2,
0.1 x 1 = M2 x 10
M2 = 0.01M (solution A)

Repeat the same process,
0.01 x 1 = M3 x 50
M3 = 0.0002 M (solution B)

Hence , [OH-] = 0.0002M
pOH = -log [OH-]
= 3.70

pH = 14 - 3.70 = 10.30

Wednesday 15 August 2012


Can someone please help me solve this.....Use the following reaction to answer the question?

Use the following reaction to answer the questions….

4Fe(s) + 3O2(g) → 2Fe2O3(g) ΔH= -1700kJ

a. How many kJ are released when 2.00g Fe react?
b. How many grams of rust form when 453kJ of energy are released?


I have all my other questions and problems solved its just this one that has me completely lost....can someone help me and explain how they got there answers? Thanks so much!

Best Answer - Chosen by Voters

From the equation, 4 mol of Fe will release 1700kJ of energy.

2/56 mol of Fe will release (1700/4) X (2/56)
....................................= 15.18 kJ

From the equation, 1700kJ ---------------------> 2 mol of Fe2O3 ( 2 x 160g)

.............................453kJ ----------------------> 2 x 2 x 160 x 453 /1700
......................................… = 170.54 g

What is the formulas of the conjugate bases of the following Bronsted-Lowry acids.?

(a) HClO4
(b) H2PO4-
(c) HCN

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A conjugate base of an acid is the species formed when one proton(H+) is removed from the acid.

a) ClO4-

b) HPO42-

What is the formulas of the conjugate bases of the following Bronsted-Lowry acids.?

(a) HClO4
(b) H2PO4-
(c) HCN

Best Answer - Chosen by Voters

A conjugate base of an acid is the species formed when one proton(H+) is removed from the acid.

a) ClO4-

b) HPO42-