chem prelim (1 Viewer)

[minijumbuk]

Re: chem prac

You sure got a lot of questions xD

http://www.boredofstudies.org/view.php?course=11

Search for "11" or "prelim", you'll find them.

 

[Continuum]

Re: chem again!!!!!

It's actually a semi-metal.
I'm not sure HOW, but yes, Boron does form weird bondings.

It's actually to do with quantum orbitals and orbital hybridisation. Boron is represented by [He]2s²2p¹. Taking into account only the outer electrons, the 2s orbital and 2p orbital becomes a 2s and two 2p orbitals (move an electron up from 2s into its own orbital inside 2p), which hybridises to form three sp² hybridised orbitals. So now that you have 3 orbitals, they push each other away according to the VSEPR theory to form bond angles of 120^o.

 

[minijumbuk]

Re: chem again!!!!!

=O

Wow dude, you're in year 11! How do you know all these stuff xD
You're doing Chemistry Olympiad?

 

[Continuum]

Re: chem again!!!!!

Yeah.

 

[12o9]

Re: chem again!!!!!

Yeah.

Oh gee. there goes my chance of doing well.

 

[x.Exhaust.x]

Re: chem again!!!!!

It's actually to do with quantum orbitals and orbital hybridisation. Boron is represented by [He]2s²2p¹. Taking into account only the outer electrons, the 2s orbital and 2p orbital becomes a 2s and two 2p orbitals (move an electron up from 2s into its own orbital inside 2p), which hybridises to form three sp² hybridised orbitals. So now that you have 3 orbitals, they push each other away according to the VSEPR theory to form bond angles of 120^o.

 

[bubblesss]

chem help!!!!!!1 please

calculate the mass of magnesium oxide formed when 10 gmsof magnesium burns in excess oxygen?

does the excess have an effect on the question?

thanks in advance

edit : sorry its magnesium oxide.

 

[tommykins]

回复: chem help!!!!!!1 please

Excess means it undergoes complete combustion (ie. reacts completely)

2Mg + O2 -> 2MgO

 

[bubblesss]

Re: 回复: chem help!!!!!!1 please

Excess means it undergoes complete combustion (ie. reacts completely)

2Mg + O2 -> 2MgO

so how do i do the question?

 

[12o9]

Re: chem help!!!!!!1 please

Mole Ratio 2:1:2

n(mg)= m/M = 10/24.31 = 0.41

n(MgO) = m/M => 0.41 = m/(24.31+16)

therefore m = 16.58g

 

[tommykins]

回复: Re: 回复: chem help!!!!!!1 please

Ah sorry, got my chemical equation wrong. (tad weird the question goes magnesium oxide when magnesium has valency of +2? :S)

Mg + O2 -> MgO2


n(Mg) = 10/24.3 = 0.4114 moles

1:1 ratio with magnesium oxide, thus we produce 0.04114 moles of magnesium dioxide.

m(MgO2) = n(MgO2)*M(MgO2) = 0.4114*(24.3 + 16x2) = 23.16grams

 

[12o9]

Re: 回复: Re: 回复: chem help!!!!!!1 please

Ah sorry, got my chemical equation wrong. (tad weird the question goes magnesium oxide when magnesium has valency of +2? :S)

Mg + O2 -> MgO2


n(Mg) = 10/24.3 = 0.4114 moles

1:1 ratio with magnesium oxide, thus we produce 0.04114 moles of magnesium dioxide.

m(MgO2) = n(MgO2)*M(MgO2) = 0.4114*(24.3 + 16x2) = 23.16grams

Why is it MgO₂ ?

*edit* nvm the question says so.

 

[bubblesss]

Re: 回复: Re: 回复: chem help!!!!!!1 please

Why is it MgO₂ ?

*edit* nvm the question says so.[/quote)


sorry questin is magnesium oxide.

 

[bubblesss]

Re: chem help!!!!!!1 please

so is it the same case without the excess in the question?

 

[tommykins]

回复: Re: chem help!!!!!!1 please

Thought so, if it's magnesium oxide follow 12o9's working.

The question would be different without the excess because we wouldn't know how much magnesium reacts with the oxygen as it is there in limited supply.

The 'excess' is there for you to be able to construct the chemical equation and use it per normal.

 

[danz90]

Re: 回复: Re: 回复: chem help!!!!!!1 please

wouldn't MgO2 be Magnesium Peroxide, not Magnesium Dioxide (as that doesn't exist) ???
lol it must say MgO in the question.

 

[bubblesss]

Re: 回复: Re: chem help!!!!!!1 please

Thought so, if it's magnesium oxide follow 12o9's working.

The question would be different without the excess because we wouldn't know how much magnesium reacts with the oxygen as it is there in limited supply.

The 'excess' is there for you to be able to construct the chemical equation and use it per normal.

so how would u do this question if the excess wasn't there?
sry for buggin u
get mixedup with this stuff

 

[bubblesss]

Re: 回复: Re: 回复: chem help!!!!!!1 please

wouldn't MgO2 be Magnesium Peroxide, not Magnesium Dioxide (as that doesn't exist) ???
lol it must say MgO in the question.

yep yep it does. my bad.

 

[danz90]

Re: 回复: Re: 回复: chem help!!!!!!1 please

2Mg + O2 ----> 2MgO

n(Mg) = 10/24.31
n(Mg) = 0.41135 mol

n(Mg)/n(MgO) = 2/2

.: n(MgO) = 0.41135 mol

m(MgO) = n * M
m(MgO) = 0.41135 * 40.31

m(MgO) = 16.58g
= 17g (2.s.f.)

 

[tommykins]

回复: Re: 回复: Re: chem help!!!!!!1 please

so how would u do this question if the excess wasn't there?
sry for buggin u
get mixedup with this stuff

You can't do it if excess wasn't there.

Be assured that they'll never give you a question where there isn't excess though.

For example - you should know about combustion right? In complete combustion between a hydrocarbon and oxygen the products are only water and CO2. However, this only takes place if there is excess of O2, if there wasn't an excess then the products would be CO and C along with CO2, we're unable to guess how much of each is produced, thus we can't really do any calculations.