AS Chemistry P2 Prep.

[Jaf]

saudha... how do u determin the moleculer formula of the compound?

Mass spectrometer!

 

[saudha]

u burn a known mass then subract how much oxygen reacted with substance and then calculate the mole of the substance to the oxygen.... like ratio by dividing it by.....if oxygen.... mass of O2/16...and the ratio thingy....right?

 

[angelgirl:)]

Mass spectrometer!

no i meant by combustion method...i no how to do it...i was just asking Saudha to tell me wether she knows it or not since we 2 r studiying together...anyways thanks...

 

[saudha]

angelgirl:) oh how do we get moles by titration?

 

[angelgirl:)]

u burn a known mass then subract how much oxygen reacted with substance and then calculate the mole of the substance to the oxygen.... like ratio by dividing it by.....if oxygen.... mass of O2/16...and the ratio thingy....right?

yes....so u no it now...

 

[saudha]

angelgirl:) oh how do we get moles by titration?

.....yah exp me this a lill ....doubty

 

[angelgirl:)]

saudha
how do u write the chemicle formula by electronic confegration and the ways they r combind with othe relements to form compounds?

 

[saudha]

saudha
how do u write the chemicle formula by electronic confegration and the ways they r combind with othe relements to form compounds?

i dint get you

 

[angelgirl:)]

.....yah exp me this a lill ....doubty

hmmm...from given conc and volume...

 

[saudha]

just like the practicals right?

hmmm...from given conc and volume...

 

[angelgirl:)]

i dint get you

total no.of +ve charges in the compound must be exactly = to the total no. of -ve charges eg complete balanced eqn

 

[angelgirl:)]

just like the practicals right?

yes...

 

[saudha]

total no.of +ve charges in the compound must be exactly = to the total no. of -ve charges eg complete balanced eqn

like Mg2+ -------------- 2(OH)-

 

[angelgirl:)]

like Mg2+ -------------- 2(OH)-

yes...

 

[saudha]

soo nxt ch now?

yes...

 

[angelgirl:)]

soo nxt ch now?

ok...

 

[user]

Assalamoalaikum wr wb!
In Nov:2009#1 Q:3 (c) why does the second peak needs to be lower than the first?

 

[Gémeaux]

Assalamoalaikum wr wb!
In Nov:2009#1 Q:3 (c) why does the second peak needs to be lower than the first?

because that peak represents the reaction after the formation of the intermediate.

 

[confused_soul]

These notes may help u

heyy dis link isnt opning

 

[saudha]

The central atom is C and it is surrounded by 4 hydrogens. First find the number of electrons being shared. In this case, it is 4 from carbon and 1 from each hydrogen, totaling up to 8 electrons. Now, each bond uses 2 electrons. There are 4 bonds, and therefore no lone pairs. So CH4 has 4 bonds and no lone pairs.

Another one: NH3. The central atom is N and it makes 2 bonds with hydrogen. The total number of electrons being shared are 5 from Nitrogen and 3 from the hydrogens. So total electrons = 8. Nitrogen makes 3 bonds (which use 6 electrons), so 2 electrons are left making 1 lone pair. So NH3 has 3 bonds and 1 lone pair.

When you have the bonds and lone pairs decided, learn this:

2 bonds, 0 lone pair = linear and 180 degrees (e.g. CO2)
2 bonds, 1 lone pair = angular and 117 degrees. (e.g. BF3)
2 bonds, 2 lone pair = angular and 104.5 degrees. (e.g. H2O)
3 bonds, 0 lone pair = trigonal planar and 120 degrees. (e.g. AlCl3)
3 bonds, 1 lone pair = trigonal pyramidal = 107 degrees. (e.g. NH3 as I did above)
4 bonds, 0 lone pair = tetrahedral = 109.5 degrees. (CH4 as I did above)
5 bonds, 0 lone pair = trigonal bipyramidal = 90 and 120 degrees. (PCl5)
6 bonds, 0 lone pair = octahedral and 90 degrees. (SF6)

The theory is basically that the electron pairs arrange themselves around the central atom to minimize the amount of repulson, so they try to be as far apart as possible. Also, you need to know that lone pair - lone pair repulsion > lone pair - bond repulsion > bond-bond repulsion. Lone pair repulsions are stronger because they are closer to the atom and therefore exert more pressure