Chemistry: Post your doubts here!

[The Sarcastic Retard]

Hello! I have doubts in question 1 of this paper

http://studyguide.pk/Past Papers/CIE/International A And AS Level/9701 - Chemistry/9701_w04_qp_4.pdf

A) Why is the concentration of H2SO4 halved in q1 a (ii)??
Mark scheme: (pH = 0.70) ⇒ [H+
] = 10-0.7 = 0.20 (mol dm-3) [1]
∴ [H2
SO4] = (0.10 mol dm-3)


B) I dont understand how to solve q1 a (iii)
Mark scheme : (end point is at 34.0 cm3
(± 0.5 cm3),
so amount of H+ used = 0.2 x 25/1000 = 0.0050 mol ecf from (ii) [1]
moles of guanidine = moles of H+ = 0.0050 mol
[guanidine] = 0.005 x 1000/34.0 = 0.147 (mol dm-3) [1]


please if anybody could explain I would be very very thankful to you!!

1a)ii)
pH = -log[H+]
0.7 = -log[H+]
-0.7 = log[H+]
10^-0.7 = [H+]
[H+] = 0.2 moldm^-3
Ratio of H2SO4 and H+ is 1 : 2
so 1 mol of H2SO4 ----> 2 mol of H+
so, how many moles of H2SO4 will be produce by 0.2 mol of H+ ? ---> crossmultiply and it will be 0.2/2 = 0.1 mol hence its halved.

iii) amount of H+ used = 0.2 * 25 * 10^-3 = 0.005 mol (When a 25.0 cm3 sample of dilute sulphuric acid was titrated against a solution of guanidine, the following titration curve was obtained.)
so now moles of B = moles of H+ used as ratio is 1 : 1 = 0.005 mol
So concentration of B = 0.005 / (34 * 10^-3) = 0.147 moldm^-3 (From graph u can read that end point is at 34cm^3)

 

[Salonee B]

1a)ii)
pH = -log[H+]
0.7 = -log[H+]
-0.7 = log[H+]
10^-0.7 = [H+]
[H+] = 0.2 moldm^-3
Ratio of H2SO4 and H+ is 1 : 2
so 1 mol of H2SO4 ----> 2 mol of H+
so, how many moles of H2SO4 will be produce by 0.2 mol of H+ ? ---> crossmultiply and it will be 0.2/2 = 0.1 mol hence its halved.

iii) amount of H+ used = 0.2 * 25 * 10^-3 = 0.005 mol (When a 25.0 cm3 sample of dilute sulphuric acid was titrated against a solution of guanidine, the following titration curve was obtained.)
so now moles of B = moles of H+ used as ratio is 1 : 1 = 0.005 mol
So concentration of B = 0.005 / (34 * 10^-3) = 0.147 moldm^-3 (From graph u can read that end point is at 34cm^3)

thank you so much for that clear explanation!! really helped

 

[The Sarcastic Retard]

thank you so much for that clear explanation!! really helped

^_^

 

[Eugene99]

 

[Catherine_1]

Does anyone have some good resources for NMR and infrared spectroscopy, please?

 

[Saad the Paki]

Basically we have to do this for all the options and check if the mass matches the one given in the question

 

[Akshajistari]

Please explain. The answer is B.

Saad the Paki My Name

 

[Akshajistari]

Koi Hai? >>__<<

 

[Saad the Paki]

Please explain. The answer is B.View attachment 59447

Saad the Paki My Name

The only difficult thing here is to form the equations, which u have to do by balancing the oxidation numbers.
Once the equation is there, find the moles of Sn2+, and then do the ratio thing.

 

[Akshajistari]

The only difficult thing here is to form the equations, which u have to do by balancing the oxidation numbers.
Once the equation is there, find the moles of Sn2+, and then do the ratio thing.

Thanks! Someone had helped in another thread already, but this makes it even clearer.

 

[My Name]

Koi Hai? >>__<<

Didn't get tag alert? :/

 

[Saad the Paki]

Didn't get tag alert? :/

Me neither. ....

 

[Akshajistari]

Challo, ab to pata chal gaya...

 

[Eugene99]

Challo, ab to pata chal gaya...

this signature of yours! Haha XD... causes a lot of trouble!

 

[Akshajistari]

this signature of yours! Haha XD... causes a lot of trouble!

 

[Syed Umar]

Heyy, I need help solving the Hess law questions and the enthalpy changes of formation and combustion etc.
All of them seem to have a different way to them. Any help?

 

[Awesome12]

Heyy, I need help solving the Hess law questions and the enthalpy changes of formation and combustion etc.
All of them seem to have a different way to them. Any help?

There is a simple concept behind them. Why not post a question?

 

[Syed Umar]

Which compound might X be? In its reaction with sodium, 1mol of a compound X gives 1mol of H2(g).

A CH3CH2CH2CH2OH B (CH3)3COH C CH3CH2CH2CO2H D CH3CH(OH)CO2H

Answer is D, but how?

 

[Syed Umar]

There is a simple concept behind them. Why not post a question?

I'll let you know next I see one..

 

[Syed Umar]

There is a simple concept behind them. Why not post a question?

What is the enthalpy change under standard conditions for the following reaction? The standard enthalpy changes of formation of iron(II) oxide, FeO(s), and aluminium oxide, Al2O3(s), are and –266kJmol–1 and -1676kJmol-1 respectively.

What is the enthalpy change under standard conditions for the following reaction?


2O3(s)(s) 3Fe(s) + Al 3FeO(s) + 2Al

Shouldn't it be Reactants - Products?
So, 3x(-266) - (-1676)= +878kJmol-1?