Biology; Chemistry; Physics: Post your doubts here!

[aleezay]

chemistry may/june 2011 p1 zone 1 Q11:


The equation for the reaction between calcium carbonate and hydrochloric acid is shown.
CaCO3(s) + 2HCl (aq) → CaCl 2(aq) + H2O(l) + CO2
(g)
How many moles of calcium carbonate will give 24 cm3
of carbon dioxide when reacted with an
excess of the acid?
(Assume one mole of carbon dioxide occupies 24 dm3.)
A 1 mol B 0.1 mol C 0.01 mol D 0.001 mol

shudnt the answer be A? the marking scheme says its D..

 

[Gémeaux]

chemistry may/june 2011 p1 zone 1 Q11:


The equation for the reaction between calcium carbonate and hydrochloric acid is shown.
CaCO3(s) + 2HCl (aq) → CaCl 2(aq) + H2O(l) + CO2
(g)
How many moles of calcium carbonate will give 24 cm3
of carbon dioxide when reacted with an
excess of the acid?
(Assume one mole of carbon dioxide occupies 24 dm3.)
A 1 mol B 0.1 mol C 0.01 mol D 0.001 mol

shudnt the answer be A? the marking scheme says its D..

according to equation 1 mole CaCO3 gives 1 mole of CO2 which has a volume of 24 dm^3
now we need to know for cm^3. as i dm^3 has 1000 cm^3, use the unitary method.
mole of CaCO3 : volume of CO2
1 : 24000
x : 24
so x will be 24/24000 which is 0.001 i.e. D

 

[snowbrood]

no problem

 

[hassankhan]

paper 41 or paper 42

paper 42

 

[scouserlfc]

can anyone tell me how to remove yeast in fermentation process and why does limewater prevents air entering the apparatus in fermentation process

Oh well i saw many answers here but what i think i s this :
the removal of yeast i dont think anyone asks but if i remember correctly the solution is first filtered and then distilled to seperate water from alcohol !

For the second problem limewater acts as a one way ! It absorbs CO2 coming from the yeast solution and prevents any gases going in the solution mantaining a constant anaerobic environment

For the removal of yeast by killing i think the people might want to save that little guy but again its not an industrial method of making ALCOHOL !

 

[scouserlfc]

paper 42

I would use the following !
Clamp stand,clamp and then suspend the magnet i have to differentiate !
i will then use iron paper clips and put them on the magnet the one which holds the most clips is the strongest !

FOr b part ! repulsion finds a magnet and the bar only attracted is the iron rod and not magnet as u must have read repulsion is the only test of finding whether a substance is a magnet !

 

[snowbrood]

I would use the following !
Clamp stand,clamp and then suspend the magnet i have to differentiate !
i will then use iron paper clips and put them on the magnet the one which holds the most clips is the strongest !

FOr b part ! repulsion finds a magnet and the bar only attracted is the iron rod and not magnet as u must have read repulsion is the only test of finding whether a substance is a magnet !

good job man

 

[aleezay]

according to equation 1 mole CaCO3 gives 1 mole of CO2 which has a volume of 24 dm^3
now we need to know for cm^3. as i dm^3 has 1000 cm^3, use the unitary method.
mole of CaCO3 : volume of CO2
1 : 24000
x : 24
so x will be 24/24000 which is 0.001 i.e. D

oh i didnt luk at 'cm3'.my bad :O

 

[Gémeaux]

 

[Alchemist4lyf]

Salam all. I have this small question, which has recently been bugging me. It's about the concentration of ions and its effect on selective discharge in the electrolytic process.
I need to know. Is the concentration of ions effect (higher concentration, first to discharge, regardless of position in electrochemical series, with exceptions) only for anions, or cations aswell. Meaning, would metals higher in reactivity series (except the really reactive ones) also discharge in preference to hydrogen or lower ions, if their concentation was higher? Or does this only pertain to anions?
Your help will be greatly appreciated.

 

[scouserlfc]

Salam all. I have this small question, which has recently been bugging me. It's about the concentration of ions and its effect on selective discharge in the electrolytic process.
I need to know. Is the concentration of ions effect (higher concentration, first to discharge, regardless of position in electrochemical series, with exceptions) only for anions, or cations aswell. Meaning, would metals higher in reactivity series (except the really reactive ones) also discharge in preference to hydrogen or lower ions, if their concentation was higher? Or does this only pertain to anions?
Your help will be greatly appreciated.

Nopes the cations are never discharged on their concentrations it just those anions that discharge,higher reactivity metals will only discharge in molten solution of their compounds !

 

[Alchemist4lyf]

Alright. Thanks for that. But I'm still skeptical on this one. I'll wait until someone else also answers, because this seems very important. Thanks a lot

 

[BEEBO2000]

I need the Nov.2011 Biology papers along with the marking schem

 

[SalmanPakRocks]

http://www.xtremepapers.com/communi...pers-marking-schemes.11543/page-5#post-155927

 

[ktc]

Alright. Thanks for that. But I'm still skeptical on this one. I'll wait until someone else also answers, because this seems very important. Thanks a lot

The general principle of electrolysis of aqueous solutions is the ions lower in the cation/anion series get discharged first.

Then again, when it comes to concentrated aqueous solutions;

The CATION (positive ion/metal ion) lower in the reactivity series of metals gets discharged first - We do not take into account its concentration within the solution.

The ANION (negative ion/non mental ion) concentrated in the solution gets discharged first – here, we do not take into account its position within the anion series.

For example;
Concentrated aqueous Sodium Chloride, the ions present in the solution will be;

Na+ (Sodium ion)

Cl- (Chloride ion)

H+ (Hydrogen ion)

OH- (hydroxide ion)

We can see that Na+ and H+ are;

· Cations/positive ions/metal ions

· Na+ is concentrated in the solution

· H+ is diluted in the solution

· Na + is higher in the reactivity series

· H+ is lower in the reactivity series

So, which cation gets discharged first at the cathode? Na+ or H+?

It’s H+, despite the fact that it is NOT concentrated in the solution.

Why?

Cuz you should remember;

1. Cations/Metal ions undergo reduction at the cathode – it gains electrons and become atoms.

2. Metals have a tendency to lose electrons.

Now the Cation reactivity series is based on the electro positivity of each metal. Sodium metal being higher in the series has a higher tendency to lose electrons than Hydrogen, being lower in the series.

Going back to point 1, consequently, how do you expect Sodium to gain even more electrons when what it really wants is to lose it?
Hydrogen, on the other hand, being the good lil ion it is, is less electro positive (it has a lower tendency to gain electrons) and therefore, referring to point 1 again, the Hydrogen ion gets discharged first at the cathode, gains electrons and becomes hydrogen gas.

Moving on to the anions present in the solution;

In a concentrated solution, it is ALWAYS the ANION more concentrated in the solution gets discharged first at the anode, and undergoes oxidation (loses electrons) to become an atom/molecule. The position of the anion is not taken into consideration – it doesn’t matter if it’s higher or lower in the anion series.

So here, which anion gets discharged first at the anode? OH- or Cl-?

Cl-

Why?

Cuz Cl – is CONCENTRATED in the solution. OH- is not.

And they all lived happily ever after.

 

[multixamza01]

Dude , you could have explained in a lot less space. I'm sure the person would have got it , they're not retarded you know! its good that youre trying to help , but if dont do it at the expense of your own time.

 

[ktc]

Dude , you could have explained in a lot less space. I'm sure the person would have got it , they're not retarded you know! its good that youre trying to help , but if dont do it at the expense of your own time.

Thanks for caring so much about my time and how I utilize it.

 

[multixamza01]

Thanks for caring so much about my time and how I utilize it.

Watever due. Was just suggesting , but if you wanna show attitude , then your choice.

 

[Ayesha Olakh]

what r the condition to produce hydrogen gas while electrolysing copper sulfate?

 

[SalmanPakRocks]

what r the condition to produce hydrogen gas while electrolysing copper sulfate?

Well the solution must be aqueous!